I'm not following how you came to this conclusion, but to give you the benefit of the doubt, I pulled up 'Superman' (2025) on both IMDb and RT as it's the one in most recent memory - and mine also. This is the result:

From my recollection, 'Superman' was serviceable fare and my personal rating is closest to IMDb's critics than any other. My first reaction upon seeing the large disparity between mine and RT's ratings (both) isn't that they're full of bots bombarding RT with perfect scores, but perhaps the people on RT simply has a preference to superhero movies.

I may very well be missing something as you seem more intimate with the working on both sites and would appreciate any insight you can provide.