I happen to notice that when units are promoted to elite the helmets are not the same. Some have Hoplite like and some are just plain helmets. I was playing Macedon and the melee cavalry, foot skirmisher (haven't tried the cavalry skirmisher), and archers behave the same way. I had 11 slingers but there was no difference. They look nice though. My opponent Seleucid archers behave the same thing.