mark_roach - 11:00 am on May 5, 2011 (gmt 0)
The fluctuation could be due the amount of data WMT has to work with. Does WMT say "These estimates are of high accuracy (more than 1000 data points)." ?
If not that could be the reason. I have another site which fluctuates between 1 and 6 second response times according to WMT because they have less than 100 data points to work with.
If you have some quick pages and some slow pages on a low usage site it is quite likely you will see significant variation.