Reasonable man theory
If a robber claims to have a weapon, and acts in a manner that suggests he has one, then it really doesn't matter if he's faking it. More to the point, there's no way for you to know he is bluffing, and it's very dangerous for you to assume that he is.
If a reasonable person would think he is armed, then for legal purposes, he is armed.
We have way too many instances where I live of robbers shooting people whether they comply or not. One of the more recent victims was an off-duty city policeman, who died of a single gunshot to the torso.