robably TimeMachine local snapshots is enabled on your Mac. Local snapshots are basically backup files stored on your local hard-disk (as opposed to regular ones stored on the external TimeMachine volume). This is a useful feature for people who travel a lot, so they can revert files to recent versions even when they are not connected to their home TimeMachine.
To disable "local snapshots" open Terminal and enter sudo tmutil disablelocal and hit ↵.
Enter your password at the prompt (the password will not be visible) and hit ↵ again.
Enter exit, hit ↵ and quit Terminal.