What was the context here if you don’t mind sharing, why one dollar and not your whole wallet? Why pull a gun?? Isn’t that a felony? Won’t police respond immediately to the report of someone threatening your life via brandishing a gun in public (more people in danger if not caught)?
They could ask the potential victim first, and then, if the value is too low, not bother pulling a gun. But all potential victims have an incentive to lie and say they only have $1. Knowing that, a rational robber would pull a gun without requesting the info first.
Regarding police responding immediately: you can't call the police until after robber has lowered their gun and/or left the scene. So by the time you call the police, you're no longer being threatened.
As for why I don't know.