Bogdan is quite correct; additionally, the Tyros 2 drum sets fewer than the T3, and the T3 drum kits were fewer than the T4 (of which all current PSRsx contain T4 drum kits; except the sx900 also contains live versions of the Genos kits).
Which means the majority of sx600 styles are designed for the T4 drum kits; nothing wrong with using the T2 drum kits on them (the mappings were the same, unlike revo, but *not* on the retro kits - house, EDM, break, hip hop kits, you'd have to manually remap yourself). The majority of new styles on the sx600 used those newer T4 kits
Mark