MSE scans 50000 files sometimes, 10000 files other times in Quick scan mode--why?

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In quick scan mode, Microsoft Security Essentials running in Windows 7
will sometimes scan around 50000 files, but sometimes only 10000
files, on the same computer, on the same day.  In fact just now I
scanned the former number a few minutes ago (running it for the first
time today after bootup) then again running it, MSE says it only
scanned 11144 files.

Anybody else experience this?

I've run it in Full scan mode and found no 'viruses' (or badware if
you will) and I get the latest signatures automatically.

RL

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