I've posted this before. Here's a copy paste.

When films are mixed for theatrical release they're mixed in surround, most often 5.1 or 7.1 depending on budget. Most 7.1 things are done in California, NYC (where I work) handles a lot more indie/doc material, our rooms arnt set up for 7.1, anyway, doesn't matter.

With both of these setups the dialoge goes pretty much entirely into the center channel (with some reverb or OS dial getting tossed into L or R, very rarely surrounds.) FX/Music occupy the surround speakers. This setup makes dialoge pretty easy to hear in the theater, as it has its own speaker behind the screen in the middle at all times.

When mixing for theater, films are typically mixed to a Dolby standardized 85dB SPL, TV is 79db SPL, at mix position (where the mixer sits) with very little overall compression or limiting. As the theaters should be relatively standardized once you mix to this level it should all play pretty fine in the actual theater provided the mixer knows what they're doing.

HOWEVER when films get broadcast on TV/DVD you very often listen to the LT/RT or just a stereo folddown as your TV only has stereo speakers. LT/RT is a very old Dolby technology that shifts the surrounds EDIT:phase polarity (phase is physical waves, polarity is electrical signal) by 90 degrees and drops them into the left and right along with everything else, then collapses the entire mix into two tracks. The dialoge track gets a slight level bump then is split evenly into Left and Right. This LT/RT tracks can then be played back either on stereo speakers and sound "relatively" like the original mix. Or be run back through a Dolby Decoder (DVD players/AV Receivers typically) and be phase shifted back into a surround mix. It's not perfect, typically affects the entire mix, and can exacerbate any badly mixed sections, also if the theater or home decoder isn't set up right (one sides amp set at a different level than the other) it can throw the whole mix out of whack.

How on earth do poorly mixed sections or inaudible sections of films occur? Surely the filmmakers want everything to be perfect? We certainly do. BUT film sound mixing stages for major blockbusters can upwards of a grand per hour for the room and mixer (typically closer to $400 on average,) plus editors wages for 3+ often union editors (Dialoge, Music, Effects, sometimes someone separate for Foley, assistants, etc.) And sometimes a producer in the room who's job is always to say "Guys we can't afford to spend any more time on this lets move on."

Add to this that unlike theaters, with the 85 dB standard. Most TV networks require far more compression and different levels. Most require either -2dBFS or -10dBFS at peak average, and a -24dBFS/LKFS (average weight of dialoge) This requires quite often a whole remastering of the show just for television. Something that can be quite expensive and often issues get glossed over or ignored because there simply isn't the budget to get them perfect.

So ultimately there often just isn't the budget to spend time on getting everything perfect for both theatrical and television/DVD, and theatrical typically has priority, so some parts get glossed over. Most often when we're printing out the mix we make the 5.1 then just on the fly fold down everything to the LT/RT stereo on the fly. We check it. But unless theres a really glaring issue the fold down through various plugins is just let to run.

It should sound okay. Except when you're trying to watch it on a tiny tv or an ipad with tons of background noise.

TL;DR, We try, If we could get it perfect on all devices we would, but budget and time combined with different standards for Theatrical vs TV/DVD often means mixes don't translate well to small TV speakers. The masters delivered to Netflix are probably the same theatrical 5.1 and LTRT from the master tapes made at the mix. Filmmakers are constantly trying to recoup costs, and I can almost gaurntee Netflix isn't paying enough for content to justify going back to a $300-400 an hour mix stage, except maybe on huge productions.