Newer Yamaha keyboards has a GS bank, what is usually hidden. MIDI files in GS may recall the GS set.
These are normal XG voices, but sorted differently.
One thing are the drum kits. The GM standard knows one drum kit only and a MIDI file strictly following the GM rules, works fine on any keyboard brand.
But most MIDI file manufacturer labels a MIDI file as "GM", but in fact it's drum track is GS.
In earlier Yamaha's you could notice, that some MIDI files has the hand clapping intro known from the song "Lollypop", because in GS, the brush kit has on the Clap key a brush hit sound. Or the drummer clicks always his sticks, the reason is, Yamaha has a sticks sound, where Roland has a snare drum.
When a GM MIDI file doesn't say to the Yamaha keyboard "I am in GS standard, please give me the correct kit", we have some strange effects sometimes.
There seems to be no way, to re-voice a GM MIDI file, when it's drum parts aren't compatible to XG.
In Tyros 4 you could program custom drum kits, what follows the GS rules, in PSR S series, you can't do it.
I found a solution, but it's difficult to explain and to understand.