If you take a look at Google's transparency report for the U.S. you will notice that something changed on or about Aug 8th, 2011. Before that date the data graphs are distinctively choppy and jagged where after Aug 8th they seem more smooth and predictable, with slightly higher peaks.
What I'm seeing is that when you reset the date display range on IE8, by pulling the date-range handle to the left, the graph takes a few moments to redraw, and during that time the earlier part of the graph (pre Aug 8th) is choppy. After a second or two, the graph smooths out and is consistent.
On FF6, on the other hand, the graph can take a lot longer to smooth out. Eventually, it appears that it does. I'm just across the Bay from Mtn View, so the IE8 redrawing is happening within one or two seconds, if even that long.
On the UK graph, btw, there is a big spike on Aug 8th that doesn't go away.