Extreme Tracking vs. Sitemeter:

We've long had both Extreme Tracking counters and Sitemeter, and their page view counts have generally been similar; from Dec. 6 to Dec. 11, for instance, they have been within 6% of each other. But from last Friday to this Monday, the Extreme Tracking counter has been 58% to 74% of the Sitemeter counter. (It's the Extreme Tracking counter that seems below the norm of what we'd seen during past weeks, but of course it's impossible to tell for sure who's right and who's wrong.) Any thoughts on the difference?

Note that these are page view counts, not unique visitor counts. The unique visitor counts differ as well, but I expect the measurements of what counts as a "unique visitor" to be less consistent among counters. Many thanks!