How are ratings determined

How are the 'points' you lose or gain determined?  There seems to be no rhyme or reason.  A person I am playing received 256 points on his rating when he won a game he played with someone rated close to him.

When I've won against a player rated 300-400 above me, I get 13-16 points added to my ranking ????

I'm a 1200 player.  I won against an 1840 player on time and got 13 points.  I know winning on time is not the same as winning by checkmate or resignation, but still ??

Can anyone clarify this for me?