That's true, but there is the special case of Paris, which is itself a département and has its suburbs (depending on how strictly you define the term) stretchin

That's true, but there is the special case of Paris, which is itself a département and has its suburbs (depending on how strictly you define the term) stretching over three to seven different départements. If you move within the Paris metropolitan area, it's easy to change département even while staying in the suburbs of the same city. Also, the large regions of France are called "régions" ;-)