Ok, I'll be that guy.

1. Testosterone promotes risk taking (http://goo.gl/s2gf4) 2. Men have more testosterone. 3. Salary negotiation is a risk game.

Now for the important part: this doesn't mean it's right. That would be the naturalistic fallacy, i.e. that because something is natural it must be o.k.

So, yes, absolutely combat this. Learn techniques to overcome the disparity. But do not, in a forum full of smart people, wonder WHY this is happening. The answer is obvious to anyone who doesn't mind unpleasant truth.