I get what you're saying, but it's not necessarily true all the time. From what point in time are they telling the story? We think of past, present, and future, but there is really immediate past, far past, present, future. If it's a person telling a story about something that's already happened, then, at the time, they may not have been able to know the information, but because they are telling us the story in the future, they likely know everything now, which is why it makes sense.
If the author was telling the story as if it were narration taking place only seconds after the events, then it makes less sense for the narration to have information outside the character's immediate knowledge. But if the author is telling the story as if it's the character recounting events that have already happened far enough in the past that they now know more than they did at the time of the specific events, then it makes more sense for them to have access to that information. Are the telling us the story of something that happened 5 seconds ago or 5 years ago?
It's why the phrase "unbeknownst to me at the time" comes up in first-person writing, amongst other things.
I think of it as immediate past tense, vs. future past tense. I could give a few examples, but I wouldn't want to beat a dead horse. Then again I'm probably not explaining this as well as I could lol.
Still, if the Author you're talking about were switching in and out of the two tenses, that would definitely be annoying. If they start the story with the inability to access information like that, then the whole story should be the same, and vise versa.