Why is the ISBN number on a book from college bookstore different than the ones online (same book)?

I am thinking about taking back 2 of my books because they are cheaper online, but they are the same book same author edition publisher everything except the ISBN number is different from the books from the bookstore?|||ISBN numbers are only given once to a book, check to see if the edition number is different, or if the one online is paperback as opposed to hard cover, that's the only times the ISBN for a title with same author changes (edition #, paperback or hard cover)...|||they sometimes do that just to make money..it's even cheaper if you get the previous edition,because not much changes though I wouldn't recommend that for a math class.||| That's what I was wondering! I just noticed that today! That is so weird! |||The ISBN is different because something is different. It could be a different edition, paperback v hardback, or something like a different introduction.|||Usually this means that the edition is different or they changed a few things. I know i bought a math book online that was cheaper and had a different ISBN number and about half way through the year the problems the teacher assigned didn't correspond with the problems in my book. So when i got back my assignments i had received a 0 but luckily she was understanding and worked with me on it. Just be careful|||They shouldn't be, I bought mine in the school bookstore and sold them back online and they were all the same. Are you sure you have the right ISBN