From the point of view of a Bible follower, the Quran would seem very different at certain points. People respond differently. This particular questioner is alarmed at the differences that he or she notices, but others might be attracted to the similarities and say: "how is it that another book can have such similarities with what they know to be the word of God. So if this is the word of God, and this other thing is similar, then what do we call this other thing?" This is a puzzle, and it's intriguing for many people.
Others could look at it and say, no, this is alarmingly different, that it has to be from Satan or something. One of the Christian fathers, Justin Martyr, wanted to explain to an opponent why some aspects of his faith were already in vogue in past societies, so he said, well, the devil inspired those people to make this kind of, you can say, mockery, or not mockery, but to do this kind of mimicry of what is going to be the truth to be revealed later on, maybe to confuse people, or whatever the case is. Once you invoke the devil, you can say anything.
One can even ask, "Okay, how do you know that that one that you hold in your hand that's not also from the devil?" Because we're talking about some unseen reality here. But if we look at both the similarities and differences, what might we make of it? On the one hand, it is true, as the questioner says, the Quran is confirming much that is there in the Bible. For example, in Surah 3, it says that God has revealed the Quran to the Prophet Muhammad, peace upon him , confirming that which was before it of the Torah. Elsewhere in the Quran, we find that the Quran is confirming what is there in the gospel, as well.
The Quran says let the people of the gospel judge by that which God has revealed therein. So there is much in the Quran that is confirming, approving, of that which is already in the Torah and the gospel. But at the same time, the Quran has in view that certain things may have been introduced into the previous writings which are not really from God.
The Quran says, for example, in Surah 2:79: , Woe to those who write the scripture with their own hands, then they sell it. They say that this is from God in order to sell it for a meager price. So woe to them for what their hands write and for what they earn thereby. So there is some indication here that people may have written things which are not quite from God.
Another passage of the Quran: They make it sound with their tongues as if what they're reciting is the book from God, but it is not really the book from God... (Surah 3:78). So we have the Quran coming down on the people who were writing, who were reading the previous scriptures, and charging them with some disingenuousness when it comes to representing the scripture from God correctly. That's one aspect.
Another aspect of it is that without charging anyone with bad intentions, it is a natural human limitation that when we try to memorize things and pass them on, our memories fail, and for a period of time, things were passed on orally. The Torah was passed on orally for generations, until it was put into writing. The gospels of Jesus, the one gospel that Jesus preached was passed on orally until the four gospels that we now have in the Bible were written. When people try to pass on things orally, naturally, they forget, and they mix things up a little bit. Even when you have things in writing, when you try to copy a written document, you will make mistakes in copying the written document. If somebody copies your mistaken copy, they will make further mistakes in their copies, and so the work of biblical scholarship now is to examine the copies, group them into families where they're similar, and to observe the differences, and try to trace back the differences to common, the similarities, to common sources, and thus to eliminate the differences.
But in producing such eclectic texts which show the commonalities, we have more than one results, more than one eclectic text being produced, and that shows a certain level of inconclusiveness in that whole process. So that's another aspect of it.
The Quran seems to be aware that through this human process of memorizing and transmitting the written documents, eventually, some errors can enter. But a third dimension is to say that the Quran is giving new instructions and chalking out a new direction for the the Muslim community, and this is why the Quran came to be a separate book, so it's not attached to the Old and New Testaments the way the New Testament came to be attached to the Old.
If you think of the progression of revelation, you have the Old Testament, now Christians come, and they have what they call the New Testament, and the two are bound together into a single volume, but Muslims did not see it fit to bind the Quran together into a single volume with the Old and New Testaments. Not that it cannot be done. It'll be a larger than usual volume, obviously.
It can theoretically be done, but Muslims did not go that route because Muslims recognized from the inception that their book was different, and one of the differences is that the Quran has new dispensations for Muslims.
When we put all of the three factors together, the problem with oral transmission, then the problem of written transmission, and then finally, the idea that the Quran is giving new instructions, then it makes sense that the Quran was treated as an independent book.
It makes sense to Muslims that the Quran, on the one hand, has many teachings which are very similar to the previous revelations to the point that many readers of the previous revelations would admire the Quran for that feature. But at the same time, we acknowledge that the Quran is very different, and these differences in the Quran may be alarming to some of our Christian and Jewish friends. Nonetheless, we ask them to try and read the Quran with an open mind and then try to understand and appreciate it a little bit more.