We can count 20 games x ~120 plyes = ~ 2500 half-moves; where were other 21.500 Hm gone?

Also I have RS2250XL in original box and there's a writing about 20.000

we will never know!

maybe the memory management was the reason? (just a guess)

and it is easy to compress an opening library, so it takes very little space. For sure, they were counting every ply in repeated openings (if there was just one different move in it)

Or maybe Saitek was afraid, that its library was too close to the Milano/Master Chess and wanted to give them a little push compared to the Centurion?