You probably have some system task running on your OS / file system which touches the files. Best ask your administrators.
I've looked at the files on the system and the last modified date of these files and they are correct. The modified date from the files on the file system do not match the ones shown in the catalog.
have a look at: Bug 15948264 : SHOW THE CORRECT LAST MODIFIED DATE INSTEAD OF THE CHANGE TIME
We experienced this issue due to incremental back ups (using HP Data Protector); we were able to resolve this by changing the "Do not preserve access time attributes" setting within the process.
Hope this helps somewhat.