Well what other option do you have when there are only two speakers? Only play it out of one?
The defaults for automatic sound mixing will almost always be wrong. And they will differ in how they are wrong from consumer box to consumer box.
When you master for 5.1, because you have the centre speaker, you can have dialogue at 100% volume, then out of left and right, you can have incidental noise, be that music or "atmosphere" also at 100%
(its been a while since I've mastered in 5.1) However, two channels of 100% volume (well 0dbu) is louder than just one channel. Which means that if you have lots of music, wind or other foley it'll drown out the dialogue.
It requires artistic choices from the sound team to make work properly.
> This means that sound can be muffled, because its drowned out by the incidental noise that comes from the left/right channel.
