icloud and watch data

My mother and father share the same apple id. But their phones are completely different. My father has had his watch for 6 months and my mother just got a new one. When looking at my mothers watch all my fathers activity data is on there. Why didn’t they set up as different watch accounts. Both of them set the icloud up as the same one. Is this the problem. Does one of them need to make their own Icloud account?