Hi Chernobyl.. In your second post you say ( above ) every Halo game uses it's own variant of under suit. The first post ( below ) all pre Halo 5 games use the same under suit. So is there some subtle detail I'm not picking up on?
I may have mis-worded things, so allow me to explain:
Regardless of the armour you're wearing in-game, the bodysuit will remain the same. For example, I could equip the Scout armour in Halo 4, and then equip the Recon armour - the undersuit will remain exactly the same. The undersuit only changes between games, never between armours in the same game.