Because pre-release the vast majority of "fans" who are able to watch it are diehards who are prone to not be negative about it (for many reasons). I'd actually consider 79% low considering the pool. And that's not just this movie, but any release.
Even with Deadpool which was getting crazy good buzz from the pre-release fan screenings, even tho while that proved true, you usually ignore them because of the nature of who most of those people are.
Well I'm a "fan" as well so it looks like I have a good chance of liking it then.