Why does Event title change to private event when moved to new calendar?

Thank you for reading my post.  My wife and I share an outlook.com Calendar which contains all the days we have her stepdaughter and all the stepdaughters events.   My wife recently added a bunch of re-occurring events for the daughter some of which show on days we don't have her so I asked her to remove them because I don't want to see the events on the days we don't have her.

if she goes into outlook.com > Calendar > Clicks on the event > and changes the event from the shared calendar to her private outlook calendar, the title of the event changed to private event

We were just curious as to why?

Thanks

Max