how do you know he wasn't trying to?

Not all kickers can kick it through the back of the end zone. Sometimes you have to angle it, in case it comes up a little short or the returner decides to take it out. Kickers are asked to help the coverage teams out with ball placement. He mis-hit them. It happens. You could take up one of your 53 with a specialist who just booms it out of the end zone every time, but do you really want to activate 2 kickers on game day?