Ah. Here is Chitika's explanation. See below or read for yourself.
Meet the iPad, with Real Time Stats – Chitika Research
UPDATE 4/8/2010 1:25 EST
Well, it appears that our assumptions in calculating iPad sales have been way off. That’s the bad news.
The good news is that Steve Jobs has given us (and the rest of the world) a second data point – about 450,000 iPads sold through end-of-day yesterday. With this, we’re able to get the proper assumptions – mostly, we need to refine our assumptions on how many unique IP addresses the average iPad hits our network from.
We should have the tracker updated with the second data point and correct calculations soon. Obviously, our 700k number is fairly far off. Thanks for the patience.
UPDATE 4/8/2010 2:00 pm
So it looks like an iPad coming through our system will hit from an average of 2.73 different unique IP addresses. As a fix, we’ll be implementing this as a modifier to the growth numbers. Should have an accurate count up soon.
UPDATE 4/8/2010 3:23 pm
We’ve updated the tracker to reflect a better statistical understanding of the IP-address-per-iPad quandary.