Even when you are there - judgement is a big factor.
The angle / proximity to a play can contribute to what you see. Sometimes the players are between the ump and the play - creating an obstructed view.
And to be completely honest - HS umpires are in short supply. Sometimes you get one that is older, less mobile, just there for the $ and wants to get out as quickly as possible. Like any profession, there are good and bad. Sometimes even a great ump has a bad day.
Hell even MLB umps guess sometimes.