* If tomatometer & audience score are within 5% of each other, you can trust the ratings to give you a decent indiciation of movie quality.
* If tomatometer is more than 15%+ higher than audience score, it means it's an artsy fartsy movie that critics like and movies don't.
* If audience score is 15%+ higher than tomatometer, it's a fun movie even if it's not oscar worthy. (https://www.rottentomatoes.com/m/old_school is a perfect example)
* Overly artsy
* Overly political. Reviewers feel the need to give it a positive review because they agree with the message, while the audience will split because they're not as homogenous politically.
* Outraged fans. Reviewers aren't typically fans of a given franchise and so won't notice if it ruins something that would irritate a fan. The Last Jedi is in this category.
* Bribery.
Doesn't happen as much anymore since they require proof of ticket purchase to review, but it still will on occasion. And it happened enough previously that they specifically had to put the proof of purchase condition in place.