I'd say being next to a lake makes a difference. I've had miserable nights sleeping near the ocean, or in the bottom of valleys where cool, damp area collects at night, even when the temps didn't seem that low.
It's funny about the ratings being based on wearing a full layer of clothing and all: I recently bought an Igneo myself, which I haven't actually used yet. The REI sales guy had quite a bit of disdain for me when I said I liked to sleep naked generally, and gave me a stern and snooty lecture about how the rating was based on wearing thermals, etc and that's what I "should" use. News to me, but also, hey I'll sleep how I want to sleep! I'm not some rookie here, was my thought. (and yeah, I know there advantages for keeping your bag cleaner by wearing thermals, except that I usually end up sweating like a pig and stinking terribly in thermals at night: ugh!) But now I see that really is how they measure them these days. And here I thought bags were getting lighter!
My old REI bag was a Helio (?) rated at 25 degrees at the time, yet the Igneo is a little lighter and rated at 19 degrees. I easily slept below freezing without a base layer in the Helio, nothing more than a thermal cap generally, sometimes socks. Now REI tells me I need a whole full set of thermals to get the new bag down to 19. That doesn't make their 19 sound warmer than my old 25. I guess all I'm trying to say is that the ratings are very relative to the person and also that though the system has changed and maybe actually become more scientific, it perhaps makes the consumer think she/he has a warmer/lighter bag than it really is.