This sounds as if you used those prenums years ago, and somehow lost track of which ones you'd used and which ones you had not.
In the case of the "6074836" book (the link I posted), there are release notes by you and an actual catch from someone else as of 2008, suggesting that the prenum was, in fact, used at that time. That suggests that it wasn't a typo by you, since if you'd accidentally activated the wrong BCID, whoever found the book wouldn't have been able to journal it.
If you saw some other BCer's screen name on the book-info of the books you were trying to register, that's a different matter; if you have screen-shots or links to those books, it'd be helpful to see them. But if it's a matter of a mixup in your used/not-used prenum list, you can double-check for the still-inactive ones by entering each BCID in the big box at the top of the screen. If a book-with-JEs pops up, you've used that one; if not, you should see a screen showing an inactive BCID under your name.