Arraging such a massive sale across every region and console must be a real pain so you can pretty much expect some discrepancies. On EU PS3 it is noticeably cheaper to buy 3-packs (1.99€) rather than buying the songs separately (2.37€).
I think Atomic was never officially released, even if you can still buy it. Maybe due to some issues with EMP. It's a shame because I'd like to play it on PS3.