Looking at visits per day on both sites, there is a dramatic difference. All other stats are pretty much the same, and can probably be chalked up to update differences. Currently, the stats sidebar on HI lists 470 visits/day, while Area51 gives 265 visits/day. Is the Area51 stat an average? Is it weighted?
The current stat on the homepage is calculated in a back-of-the-envelope way that isn't nearly as accurate.
The Area 51 stat is more accurate because it is pulled from the actual Google Analytics -- you should consider that definitive.