Strange package checksum report

[Garrett Wollman]

On some of my machines, I've been noticing the following in the
nightly security mail:

Checking for packages with mismatched checksums:
p5-XML-SAX-0.99_2: /usr/local/lib/perl5/site_perl/XML/SAX/ParserDetails.ini
python27-2.7.9: /usr/local/lib/python2.7/UserDict.pyc
python27-2.7.9: /usr/local/lib/python2.7/_weakrefset.pyc
python27-2.7.9: /usr/local/lib/python2.7/abc.pyc
python27-2.7.9: /usr/local/lib/python2.7/codecs.pyc
python27-2.7.9: /usr/local/lib/python2.7/copy_reg.pyc
python27-2.7.9: /usr/local/lib/python2.7/encodings/__init__.pyc
[ a bunch of other .pyc files elided ]

Does anyone know what causes this?  Force-reinstalling the package
fixes it, but only temporarily -- by the time the next security mail
comes, it's back in this state.

-GAWollman