See here for the underlying problem: Imdb ratings doesn't update even when I refresh meta data
Plex does not actually hit IMDB or RT directly for ratings, they have cached metadata and ratings stored at “http://meta.plex.tv/” which it looks at instead. Sadly those ratings are often incomplete or out of date.
Unless Plex keeps this in sync then there will always be some discrepancy with IMDB or RT no matter how many times or ways you refresh the metadata.
Incredibly frustrating, but I hope this helps!
