Retirement funds can be legally organized, in the US, in so many ways a general answer is impossible.  

But, if your ready for this one, the US goverment, in its wisdom, allows corporations to put their own stock in their internal retirement plan.  So when they buy back shares they also increase the estimated net worth of that  fund as well.  

She believed in nothing; only her skepticism kept her from being an atheist. -- Jean-Paul Sartre