Why do QoS and QoS(PS2) have different leaderboards? PS2 runs are allowed on the QoS leaderboards after all..

The PS2 version is a completely different game than the version released for other systems. As for why the other QoS leaderboard has PS2 listed, I think that's a mistake.

Gamecuber added PS2 to the platforms (along with DS), presumably he didn't realise they were entirely different entirely.