Here's a quick, somewhat technical explanation of what happened, written by
the engineering department.
Several months ago we upgraded our servers for the usual reasons.
Faster, bigger, better.
Our previous usage gathering depended on an "event" the
occured in the web server called "Session_onEnd".
When your session was over, this event was triggered and we would
record your usage.
The new Microsoft server operating system we upgraded to seems to have made
"Session_OnEnd" completely unreliable. Sometimes it happens, sometimes not.
When it didn't happen, your usage for that session was not recorded.
If you do a google search on "Session_onEnd not working" or something similar,
you will see many other programmers discovering and lamenting that this feature,
which would be so convenient and useful, is in fact not reliable.
This is an acknowledged Microsoft bug. A hot fix is available, but we
have not been able to get it yet.
Surprisingly, it was working on our earlier servers, at least well enough that
we could not detect the error. In the new servers, it was off enough that it
was easier to detect.
To fix this we rewrote some of the programming that was involved.
We do still have the older usage data, but it is not accurate and
prefer to not display it.
Your usage is higher than shown in the older reports.
Microsoft Blog - source of following quote
"Session_OnEnd not firing after applying Windows 2003 Service Pack 2
We very recently discovered a bug in classic ASP (asp.dll) you get installing the Service Pack 2 for Windows 2003; removing the SP2 makes it work again as expected, so this is a regression bug introduced by the service pack itself. Basically with SP2 installed, the Session_OnEnd() method in classic ASP is not fired anymore, thus breaking all applications which rely on that event to run cleanup code.
We already have some reports from customers, either directly to CSS or in Newsgroups. I know my colleagues are already well on this, a private fix has been produced and is being tested internally (as far as I know it works) and we are writing a KB article to document this."