Yep, like Mrprez said, it's got more insulation in it than normal (sorry about your arm there...) What can I say? I live and camp in Minnesota in the fall/early winter. I needed it!
It didn't occur to me before that I might be misleading someone that their Potomac would keep them just as warm as mine. I don't want to do that, and I don't want Mrprez to have to do the explaining to pissed off customers because of me either.
At the same time, I'm pretty darn sure that I could take this into much lower temps without discomfort, or needing added insulation. Although the standard Potomac doesn't have the extra layer, I suspect that I could probably deal with less insulation in the same conditions as last weekend.
That probably doesn't apply to everyone. I'm a very warm sleeper, which is another thing to consider.