At the time of the film's release, you only needed to look around places like Twitter and Facebook to see how the RT was manipulated. Heck, I've actually encountered people on social media who claim they are partly responsible for the movie's low audience score. Even if you think there's no way that could've been the case, there was definitely a campaign to lower the score, which largely succeeded. Also take into account the fact that hardcore fans are much more likely to rate the movie on RT than the casual fans/audiences who vastly outnumber them. Audience polling surveys like Cinemascore, comScore, and SurveyMonkey are much more reliable when it comes to the percentage of people who liked/disliked the film.
In all honesty, the way many fans reacted after The Last Jedi was frankly embarrassing. Not liking a movie is one thing, and that's perfectly fine, but what happened goes far beyond that. The movie isn't perfect, not by a long shot, and there are some legitimate arguments for the problems with it. But the utter vitriol that was spewed by the anti-TLJ crowd toward the film and its creators, especially Johnson and Kennedy, is reprehensible and inexcusable.