Wirelessly posted (Mozilla/5.0 (iPhone; U; CPU iPhone OS 4_3_1 like Mac OS X; en-us) AppleWebKit/533.17.9 (KHTML, like Gecko) Version/5.0.2 Mobile/8G4 Safari/6533.18.5)
I think it's because they pay royalties for every song played and the ads shown on the desktop declines as you reach the 40 hour limit and the mobile version shows a consistent number of ads, allowing them to play more songs.