One type is self aware. They know there are other people on the trail that may be moving faster than them. They are listening, and staying alert to let people pass by if they come up in the rear.
The other type has no idea you would like to pass them. No idea you have cut your pace in half to be in lock-step with the group in front of you.
Interestingly, the higher up you go, the further from the parking lot you are, the more likely you are to encounter the first type. Etiquette, like many things is often learned and not taught.
The more miles you go, the more you see and the more you know.