You’re selecting a primary ratings source, not an exclusive one. There are some titles for which Plex doesn’t have Rotten Tomatoes ratings; in my own library I have several (Ice Age, for example). In that case it falls back to one of the other available sources.
You can check some of your movies which are missing RT ratings against https://watch.plex.tv to see which ratings sources they have available. If there’s a discrepancy, you might try the Plex Dance for the item(s) in question. Or an individual metadata refresh.
