Not sure the %. I am guessing it's just that, a comparison of wherever it is at a given moment compared to the last reset. I doubt it has any idea of an actual pressure, just that there is or isn't a big change.
When winter hits it does go off. Not such a bad thing though, because your pressure is in fact lower and you should fill the tires back up. It's annoying, but it's "right" for lack of a better word.