Youtube lists private/unlisted videos in the Playlist counts, but does not allow access to these videos for viewing or access using tools such as yt-dlp. For this reason, the count of videos in a playlist that you see in the Youtube UI may not correspond to the number of videos yt-dlp can retrieve.
The most likely explanation for what you are seeing is that some videos in the playlist in question are private. The only way to know for sure is to look at a list of the videos in the playlist and compare that with the list of videos yt-dlp downloaded.
You could also build a script using yt-dlp that exports a list of all the videos it can access before it downloads them, and then goes through that list a second time, downloading each. This would have the benefit of giving you a log of the videos available and those downloaded and showing any errors experienced during the process.