What does "ls -l" show for the two files?
It's a bit more complex than just "basic." The USB flash drive is probably formatted FAT32, which cannot preserve all of the Linux permissions. On Linux you cannot give a file a GID for a group you are not a member of, so the GIDs may not match even on files you just copied. When rsync-ing to/from a USB flash drive, I always include the options "--no-g" and "--modify-window=3602". The "--no-g" will ignore GIDs, and the "--modify-window" avoids issues with time resolution (2 seconds on FAT32) and changes in Daylight Savings Time status (FAT32 uses local time in its time stamps).