Some games have letterboxed parts in sections of games, sometimes throughout the game. Sierra games for example have this on occasions, with some parts using all the screen, others not.

Could be because of roots from an Amiga / Atari / Console port?

It's Mode X trickery. I never seen Rayman letterboxed on a real system.

Well, at least the Ykhwong's build do the trick.
By the way - very-very hard game...