Ciao ​@Heleneb. Have you noticed if before changing the privacy controls the mileage grew correctly? I don't know exactly if it's a bug but what the followers see surely depends on privacy controls. 

https://communityhub.strava.com/insider-journal-9/profile-privacy-controls-explained-1518?tid=1518&fid=9

Thank you for your response ​@Vincenzo !

I’m not quite sure when it happened, but after changing it back to “everyone” it stills shows uncorrect stats.. around 300 km difference…

Ciao ​@Heleneb. Probably the stats are not considering all the activities. Have you considered to contact the support team opening a ticket? It depends on how many activities have you uploaded to properly check them...

​@Heleneb - Are all of the activities the same type of activity or a mix of different types?  Also, have you made sure that you changed the visibility of each and every one of those activities to viewable by everyone?