Yeah, see, that's true of me too -- I was actually a little surprised when I was uploading some of my old fic to AO3 to discover that I used to occasionally write fanfic in 1st, because it seems really awkward to me now. And yet, in original novels, I seem to default to 1st; I have trouble getting myself to write in 3rd, even though I think I have a slight preference for reading in 3rd.

It's confusing.

I think at least part of it is that some fandoms have a very strong preference for 3rd -- SGA fandom almost never uses 1st person, but I've been in fandoms that weren't that way.