It may simply mean that the specific item that was equipped (hood + robes from the pic) came with a "regenerate magicka faster" enchantment. However, it may mean that armour reduces your base magicka regeneration (meaning you regen faster in cloths than armour), and this might also serve as a distinction between light and heavy. No idea really, but, presuming they did not mix up an enchanted effect with an in game effect, it is nice to know there is a reason for mages not to ware armour (and that by going with the known heavy armour perks, this is not going to be removable if so). Hope this info is right, and new!