Why does sirius radio get different songs then fm radio?

Whenever i hear a song i like on Sirius Hits 1, i will look it up on itunes and it wont be in the top downloads, and it will have like 2 reviews. How come there are songs that are always played on sirius that arent on regular radio? also, how come i was able to vote for Pea by Katy Perry in the 20 on 20 countdown? isnt firework the current single?