Because it compresses it more. People who go by the "number of songs" it can hold should be shot. Actually, that's a bit extreme, people are just ignorant and being fed shit by Apple/Creative/Sony etc. If you use 64kbps wma, yes, you can store twice as many songs as at 128kbps MP3. It'll sound shit though. Seeing as everyone just uses MP3 anyway, just go by the size. Right click your music folder and go properties. Find out how much is there. See if it'll fit.
Anyway, it's too late, you've already bought it. I think that's the right choice though. But why did you get 2?