WMP might be sneaking some additional files over if you are telling it to ‘sync’ and there have been more files added to the computer siince you transferred songs the 1st time. And it doesn’t necessarily mean it’s just song files; it could be photos, videos, anything.
Check the INFO under the SETTINGS menu on one player with the card inserted vs. the other (also with same card). The INFO screen will show you the total number of songs (both int. & card). Are the counts the same?
I don’t mees around with WMP or MTP except to use WMP to rip CD’s. Otherwise, it’s just too sneaky for me.
MSC will set you free!