I'm kind of thinking #1, that the lightest-weight things were reserved for chemises and the like, but I don't have enough data to reasonably defend this theory.

I remember people saying, at one point at least, that the weather was cooler in the middle ages (although I think I might have heard contradictory info later), so on that front your theory would make sense. Your average person would have probably been wearing less lightweight fabric just because they wouldn't have had access to fine fabrics, right? Also, could it be the weave of these fabrics that is the problem?
Okay, those are my random thoughts. I miss sewing with you guys. It was way more fun than doing similar things alone.