Sounds like you have Explorer looking at something other than the file's actual modification date. Maybe some internal metadatum is being interpreted because the folder is considered to contain images rather than files.
Try this: In Explorer, right click on the folder in which the files live, choose Properties, then click the Customize tab. What do you see in the "Optimize this folder for:" field?
If you change it to General items, then look at the modification dates, do you now see what you expect?
-Noel