You may have one that is a mismatched duplicate.
Filter for Duplicates and see if you have an extra one:
I know how many I ‘should’ have… when an extra one shows up - it’s go go gadget…lol
Identify Duplicate - Split Apart - Fix Match on the one that isn’t what it should be. Get Info shows your file name - to identify the file to Fix.
Also note: The Roku counts a Collection as a Movie to further confuse and infuriate. Take a look at my Movie Library that says I have 177. There are 177 Collections Visible. There are over 5000 Movies and If I want to know how many I filter for Folders - 'cause each movie is in one.
In short - it’s difficult to get an accurate accounting between a file manager and a Plex Page. There are a lot of moving parts.
