Maybe somebody miscounted?
FWIW, I think I tried to order that same book on Monday and it was sold out.
I could be wrong, it could have been something similar but not the same. But I remember it because we've been pretty weak on history/soc. studies this year (heavy on science), and a lot of what I really wanted from the sale was stuff to help catch us up.
I have often ordered by Internet from various companies (not Scholastic) and had something wind up being out of stock and canceled very late in the process. I suspect that's all this is. (At least I hope so!)