Rigor Mortis Question?

I'm studying anatomy and physiology and my lecturer told us about rigor mortis yesterday. We've learned about muscle contraction and how it happens etc. and from what he said i know that rigor mortis occurs about 4-6 hours after death. I know that this involves the body becoming stiff but i was confused by an example he gave us, of a body being pulled out of a river that had the arms stiff out the front (like a zombie). if the body was to stiffen when someone had died wouldn't the muscles stiffen in a relaxed way such as arms simply flat at your side? why would your arms go out in such a position? and if the arms do that, why wouldn't the legs!? can anyone explain this?