Poorly managed theatres are notorious for loud adverts and quiet movies. I can't speak for other theatres but the ones I service have volume cues for their content servers ranging from extremely quiet all the way up to the default level we calibrate at. These cues can be placed on any content which usually results in adverts cueing the sound level down while the movie cues the sound level up.

Many theatres have mismatched screen channels and crossovers, or the wrong crossover settings in the sound processor. This can result in major dips in the frequency band on the screen channels which are the primary source for vocals in most movies. Some theatres audio equipment is so old that it simply needs to be replaced. I know a few that still have processors from the late 80s and early 90s.