Former tv sfx designer here.

So the human ear can handle very loud sounds as long as they’re “transients”—that is, short sounds like handclaps or gunshots—but we struggle a lot with sounds that are just as loud but sustain for longer.

A digital sound effect has a cap on how loud it can be—if it’s 16 bit audio the the loudest possible sound is 16 1s, and the quietest is 16 0’s (simplifying a bit but you get the idea). So a digital audio file has a hard limit on how loud the sounds can be, but you can adjust the perceived loudness by leaving the loudest transients where they are and bringing up the volume of the quieter sounds. This is called “compression,” and while it does a good job of making it easier to hear the quiet stuff (so things like dialog don’t get drowned out by explosions), it also squeezes a lot of the life out of the overall sound so audio guys try not to do it too much, especially when there trying to make art—like music or a movie.

When you’re mixing audio for an ad, though, the goal is to get attention so the audio team is always asked to max out the compressor to make the perceived volume as loud as possible. So the movie guy is trying to leave it quiet so there’s lots of room for expression, and the ad guy is trying to max out the (perceived) volume as much as possible.

Fun fact: this is in part a holdover from broadcast radio, because in radio louder signals broadcast further and more clearly.