that site sucks and lags ym computer (ty useless javash*t countdown)
it could be veolcity/dynamics. ive had experiences where a chord would blend in the mix, then the next one would stick out like a sore thumb. it was especially bad with compression which seemed to raise the higher chords even MORE. just depends on the notes played and how they stick out in the mix sometimes, with very little to do about it except automate the volume.
older keyboards get worn out. its the same not that gets loader so i assume this is what is going on.
i have two older keyboards an alesis and a yamaha that do the same thing, no matter how soft you hit one key it will hit harder then you can even hit the others. its a midi thing on one key no matter what the velocity is at max. it just happends to work for this song.