Hi Guys,
it's not only the sounds, the style player has a lot to do with it ie the note transposition tables & settings.
I look at some of the earlier Yamaha style ie psr2000, psr 1500, the styles were really simple styles, ie I mean the notes used in creating the styles.
Pull apart a newer psrs910, or T4 & things have changed. I'm fairly certain Yammie must have added additional note transpostion tables/settings along the way to be able to play the newer more complex styles. (apart from anything to do with megavoices)
Some of the earlier guitar tracks the chord voicings were recorded like you'd record piano chords.

For Korg. In one of the updates to the PA800 , they added an additional setting called CHORD which creates a smoother transition when changing chords .
Guitar mode tracks (to me at least) sound more realistic for strums than midi tracks. I don't know how authentic they are, I'm not a guitarist.

So for me, I don't beleive it's just the newer better sounds, it's the capability of the keyboard to play & accurately transpose the notes to play the newer more complex styles.