It's easy to over-complicate this, but here goes: as an indexical image medium, cinema does indeed capture reality, can capture things that exist in the profilmic reality or the existential world (Godard's quote about Breathless is that it's a documentary on Belmondo and Seberg), but yeah it's always incomplete, as language always is, and it's always shooting past whatever the intention was. So part of what cinema is about is about this gap or tension between levels of reality. This is as true of realist filmmaking as it is in the highest or most outlandish of fictions. And it's true of your everyday perception too, which is always guided by a million factors that keep you from experiencing pretty much anything objectively.

So when we say that a film captures reality, really it's a basic layer of it we're speaking to, the layer that Hawks captures by leaving in that shot in Gentlemen Prefer Blondes where Jane Russell ends up in the pool by mistake, or the one the Zapruder film got when JFK was killed (talk about an incomplete but absolutely invaluable record!).