I have to assume you are speaking of local umpires. Part of my summer was spent reviewing the plate jobs of CWS for a clinic we will put on locally. I don't recall seeing a pitch one third in from the inside corner being called a ball, nor did I see a pitch 2" into the opposite batter's box called a strike. (8" outside)
There is no doubt pitches are missed. There is no doubt that, as with players and managers, some umpires are better than others. There is also no doubt that there is no perfect umpire.
When I see an umpire call pitches off the plate a strike, what I notice most often is an error in mechanics. When I see a pitch missed on the inside corner, what I notice often is the umpire was set up in such a way that the catcher took the "slot" away and the umpire didn't see the pitch as it arrived at the plate.
When that happens, we are trained to call the pitch a ball and suggest to the catcher that he give us some room to see the pitch. This happens at all levels from the lowest amateur to the highest professional.
When an umpire's mechanics don't allow him to see the pitch properly, this is most likely an error, not an itentional act.
Like different batters can miss the same pitch for different reason, different umpires can miss the same pitch for different reasons. Regardless of the cause the result is usually losing the ball momentarily and when that happens, the brain connects the dots and gives the umpires the perception that he "saw" something that he didn't see.
With training, a lot practice and many, many "reps" umpires are able to greatly minimize the number of times this happens, but it happens.
Another thought: I don't know why people are so surprised D-1 make mistakes. Those of us fortunate to have worked our way to this level understand that we are not at the top of our craft.
Umpires in D-1, as a group, are pretty much like the players and coaches at D-1. There are a few really, really good umpires, there are some more who are better than the rest and then there are the rest, who can perform better than most umpires at the next level down. But none of us are the equal of professional umpires with the same length of service.
We don not get the time in spring training, fall instucts or winter ball. Most of us have not even been to proschool. We do not get the detailed evals or feed back.
Now, none of this is to excuse bad calls. While there may be a reason for a bad call, there is no excuse. We take this job on knowing that people who have never tried calling 90 mph pitches often expect us to be better than we or most anyone can be. That's okay, it comes with the territory. We often expect more from coaches and players than they can give as well.
But, bottom line, few umpires intentionally screw up. Most missed pitches are just that missed pitches. As a group we haven't reached the 95% accuracy that the MLB umpires have, (although many think they have). But we do fairly well.
We will become uniformly excellent about the time all nine spot batters hit .400
As long as players, managers and umpires are chosen from the population at large, even after screening and development, each group will have some members not as good as others.
Have a great off season.