It's constantly corrected with real figures,
No. It's constantly "corrected" with 1 unit every 5 seconds (for the Wii) for instance, which is not a "real figure". For every week in between "real numbers" the Wii sales is a real figure all of 5 seconds, and then for the next 600 thousand odd seconds, it's just a meaningless arbitrary number. Surely noone was expecting that the "realtime" update was more than a "guesstimation" based on previous numbers, right?
The point is that it wasn't based on previous numbers, so much as arbitrary ones. Google is doing just that for its estimation of offered GMail space, I see nobody crying and shouting.
I somehow doubt Google estimation of GMail space is just adding another MB every 5 seconds, and then recalibrating the system every month or so. More likely they're using some queue theory algorithm to accurately determine the amount of disk space needed, at any given time, based on for instance a Poisson distribution.