It sounds like you see your non-oDesk work history, but you do not see such history on the profiles of many other contractors. I do not know why that is the case and can only conjecture that there is an algorithm that stipulates when to hide or show non-oDesk work history. There is certainly no button or option that I am aware of which controls this.
My own profile is hidden (private), so I don't know what would be seen if it were visible.
I'm also noticing that freelancers claiming to be designers don't have portfolios. I don't know if oDesk is hiding portfolios or these freelancers don't have the common sense to realize that clients will want to look at examples of their work.
I bet they have not posted portfolios. I have not spent much time compiling a portfolio. Been too busy working to do so. But I have definitely never seen my portfolio hidden.