COR you said "Your own rewards can be *calculated* from the network hash rate and your own hashing rate, that is the fraction of each block that
you should get, for example if your hash rate is 1/50,000 of network hash rate then you should get an average 2 coins for every block, so 4 coins per minute. (100k coins reward and 1/2 minute block time)."

Isn't that a bit of an over simplification as some blocks are found quickly while others take a long time so the no. of pool shares and pool shares taken into account (depending how the pool works out how many shares to use) varies quite a lot.
It could take a while for any variance to average out.