Difference of elements between source directory and Plex library

[Taillandier]

Good morning
A little question about my film library.
When I check the number of films in \\TNAS-69C7\taillandier\films, I have 739 items!
When I go to my PLEX library I see 710 items, which makes a difference of 29 missing items ??????
Apparently this does not come from the type of files (AVI, MKV MP4 works very well!
Any idea why?