Google's weird math

I did a Google search for the word "blog", limited within the past 3, 6, and 12 months. The number of hits for each time period are as follows:

In the past...	Hits
3 months	20,400,000
6 months	19,400,000
12 months	21,900,000

What's wrong with this picture? Well, the hit number for 6 months doesn't make any sense. How could more pages have been updated in the past 3 months than have been in the past 6 months? Is there something going on that I'm not getting?

UPDATE (01/14/2005): I just ran the search again today and the numbers are different this time (expected), but a discrepancy still exists - this time in the 6 month vs. 12 month results. 3 months :20,300,000 6 months :22,200,000 12 months:18,900,000 I'm just baffled as to what mechanism would yield these results.