Because of the credit roll. Some movies have a longer, some have a shorter credit roll. Many/Most users won’t sit through it and abort playback when credits are starting to come up.
90% was considered a practicable middleground which covers most titles.
Until Plex can determine the exact time stamp in each individual movie where the credit roll starts, this workaround cannot be improved unfortunately.
If you have opted in to this feature Discover Together Beta : Announcement & Feedback , you should be able to see your whole history (i.e. plex.tv and your own server and your friend’s server[s] combined) on this page: https://app.plex.tv/desktop/#!/profile
