There are quite a few helmets that are the same weight but provide different armor values. This occurrence is most prevalent with the lower armor level helmets but even exists at the upper levels with items like the Phrygian Helmet with Faceplate. In a game that is supposed to encourage people to make and customize a character, it seems counterintuitive to discourage people from using certain helmets by making some statistically superior.
What is the rationale for having helmets with the same weight but different armor levels? Realism? Because when we are talking about differences in .1 weight, I don't think realism is impacted. Yet, on a game level, it does have effect. It makes it so that when I use items like the Norman Helmet or Roman Helmet, that I have to make a choice to purposely have 1 less armor than I could be having for the same weight amount.