Maybe the latter. I have done some umpiring and I can tell you sometimes those kinds of games you might - on a close one- be influenced by your state of mind :-) Umps do that all the time. For example if the catcher blocked my view the call will go to the batter - of course unless it's obviously a ball. Similar thing applies to a high tag on a close one.
Humans are not perfect and that's what makes them so interesting - and the game of baseball.