If you're still here --- or checking here --- I can explain. Velocity sensitvity is the culprit. It is mapped to the brightness of the patches on the DX7. Your Korg Trinity sends a larger number of values than the DX7 keyboard making the DX7 seem brighter when played from the Korg keyboard, If it really bugs you there may be a way to restrict the velocity values that the Korg sends --- check the manual.