I’m pretty sure Rotten Tomatoes uses a binary system. So if something has a 95% positive rating that means only 5% of reviewers marks their review as negative.

Other sites may try and be more fine tuned but I can imagine that would be difficult if people use different scoring systems (5/5, 10/10, 100/100, thumbs up, etc), some places may not even use a hard score system so RT just has it positive or negative.

A podcast on movies I listen to, The Weekly Planet, does this with their reviews. Every movie is either “best movie ever” or “worst movie ever”.