Sometimes modification time of a file which is not downloaded on computer A, but modified by computer B is not reflected immediately to computer A.
Henceforth, backup software running on computer A will think that the file has not been modified. This is a known problem in file synchronization. Also, some applications modifying the files revert or protect the mtime of the file for reasons. They are rare, but they're there.
Sometimes modification time of a file which is not downloaded on computer A, but modified by computer B is not reflected immediately to computer A.
Henceforth, backup software running on computer A will think that the file has not been modified. This is a known problem in file synchronization. Also, some applications modifying the files revert or protect the mtime of the file for reasons. They are rare, but they're there.