Why Did He Respond to Them but Not Me?

So i was outside of the building and the door was closed on the inside and this guy on the inside opened the door for me. I said thanks and he didnt say anything back, he seemed shy (which is fine but keep reading). Then 5 minutes later this person showed up and had the same issue and the same guy hellped him and the person said thanks and he said you're welcome. What does this mean, you can call me nitpicky all day long