It does. You have hit on what I've been noticing but haven't yet been able to express. Fanfic does appear to be more directed, which makes sense given that any given fanfic can focus one element of a show that has multiple strings running at any given time. And it also gives some ability to explain why a show appears to have loose threads --- the writers may have greater interest in one thread over all others --- other than just blaming the writers for being sloppy with continuity, although I personally think that writers (all every once in a while, some all the time) do tend to get sloppy in that regard.

I guess there's no reason why the human tendency to deal only with what you want to deal shouldn't apply to professional writers, but I personally have higher expectations from professional writers than "they should write what/how they like and I should be okay with that.." Coherent, entertaining plot writing is their job, and it's not like it's impossible to think ahead of time that subject X may create problem Y but that it could also conflict with solution C. But, yes, the focus attention one uses in fanfic reading/writing may well have the potential to make one hypersensitive to discontinuity and make it more galling to see.