This all stems from wargames of the 70s and 80s, when classifying the French forces in the 1815 campaign, making a distinction between on the one hand the 1st and 2nd Grenadiers and Chasseurs a Pied, and on the other hand the 3rd and 4th regiments of the same. Every rules designer knew and had to reconcile two things:
(1) They all believed implicitly that the Old Guard could never, ever rout - they might die to the last man but they were incapable of fear; and
(2) The battalions of the 3rd and 4th Grenadiers and Chasseurs that made the final French attack at Waterloos _did_ in fact rout, albeit under circumstances that might well have caused the same result in any other troops on the planet.
The appearances were saved by classifying the 3rd and 4th regiments as "Middle Guard", despite the fact that they were not actually so designated and that the true Middle Guard infantry regiments - the Fusiliers-Grenadiers and Fusiliers-Chasseurs - were not reconstituted for the 1815 campaign. Those are the fellows you're seeing in the earlier campaigns with the plumed shakos (supposedly at one point one of the colonels of the Fusiliers made a pitch for putting his lads in bearskins, to which the Emperor replied not just no but hell no).
Warlord has apparently decided to throw a sop to the old school by designating the 1815 Guard Chasseurs as "middle".