Fundamentally a documentary is trying to sell a narrative using documentary cinematic style. Now sometimes this is mostly benign or even "good." Such as detail

Fundamentally a documentary is trying to sell a narrative using documentary cinematic style. Now sometimes this is mostly benign or even "good." Such as detailing an event in a in-depth matter-of-fact style. Works such as the early Planet Earth series are simply trying to capture and show the wonder of life around the planet! And maybe hopefully encourage you to try and protect them as a secondary upshot. As time has gone on, they've produced works are that much more blatant with their "save the environment" messaging. However, most of the most popular ones have a significant editorial bias. Heck, one of the first landmark pieces, Nanook of the North, was almost completely fabricated! Often quite easy to spot as well. Some are very up front about their bias, such as Michael Moore's work. Some less so, such as the editorial narrative in features like Forks over Knives or Blackfish. And some it's quite subtle. The recent Making of a Murderer comes to mind where certain facts weren't presented and it's framed in a very sympathetic light towards the defendants. Sure they have a lot of true facts within them. And often you learn quite a bit! But they are often presented in a particular fashion or certain points are talked up while others are ignored or not presented. It's an unfortunate thing, but even the most innocuous of documentaries have some small bias. And it's up to the user to try and understand how much is narrative editorializing and how much is "truth." It's all part of making the documentary feel more true and tap into our need for a strong narrative arc via romanticizing the story we see unfold.