Generally, they are not paying for the whole or part of their journey, such as entering a station without a gate line without tapping in, then tapping in at validator such as those on the platform in Farringdon for those switching between paper and contactless, before exiting at a later station. To the system the fares look valid as there is a tap in and out which is why the card isn’t blocked, but TfL has a system called ITAP which can pickup on things like this.
There are also serial refunders as well, which is covered during one of their TV specials.