The average US border official is just like the average official(and non-official) anywhere; mostly bored, waiting for their workday to be over so they can go home and do the things that they actually enjoy.
Not to say they don't make mistakes or don't fully read/understand their systems. A co-worker was former LEO, had his id stolen at that time. We were traveling back from an a vendor site across the border, the border agent misunderstood the system alert from "double check this guys docs to make sure he is who he says he is" to "Anyone using this id is traveling under a false name!" So he got held and questioned, called his former supervisor, CPB supervisor came and showed him how to get a travel redress number.
Amusingly, since we were traveling together, I got his luggage off the carousel and kept it with me as I watched them question him. After some time they decided to look at his bags and came looking for them, I told them I had them and he was with me. They took the bags and didn't even want to speak with me.