If there were two Enterprises in the future, and week to week we never knew which one we were watching, that could explain a lot of inconsistency.
Oh no now there's at least 2 Enterprises and probably infinite Enterprises actually.
Now there's more TT problems with the episode. My live is over!
I suppose you could say the Enterprise kept replacing itself as it moved back in time but then the Christopher and Sergeant who were on the Enterprise should have replaced their old selves with that logic.