Human behavior is usually used in a generic sense; like 'human nature' and so on. As something that is typical and applicable to all of us at least in theory, rather than 'behavior that some humans do'.
Where did you take such "usual" usage from? Would you mind a citation? It surely isn't a common usage among economists: such study wouldn't be of interest to almost any of them.
I take it from the fact that I've never really seen it used in a different way before; and because that is what I take that term to mean when I see it used. The behavior of humans at an extreme basic level (the level at which we are all 'the same' rather than at the social) in other words. As it happens, I also think that it is a completely useless term as you can't separate humans from the societies in which they live in.