I'm curious that extremepool.org shows I have a Total Paid, but my wallet shows a zero balance after refresh. It hasn't been very long, but my experience with XMR on a couple different pools was immediate.
I'm further curious as to why the block reward is not always 320000, but usually some much smaller amount.
if you don't understand why the block reward isn't always the same read the op or the white papers it is explained..
as far as not receiving coins yet once the pool pays them I have been told the block chain is taking an extremely long time to sync up. I transferred some coins to poloniex earlier today and took about 6 hours.
Thank you for attempting to provide some answers, but I'm afraid my curiosity is not yet satisfied. I found an obscure link in the OP,
http://ducknote.org/aboutduckNote, and in it one obscure footnote that says the block reward can vary, apart from halving: "*Supply chart has an accurate, but approximate values, because block reward for some blocks can be lower than base reward due the big sized block penalties."
That's not much of an explanation; a proper explanation would address why large blocks are penalized, what the reward variance can be, and how often it might be expected to happen. By observation, the answer to both the latter two is "a lot".
You say "white papers"--could you perhaps provide more clues as to where additional writings reside?
As to block chain sync being the reason for non-existent or slow payment, I don't think so. My wallet and the pool are reporting the same block height. I think that extremepool is not actually paying out right now, and that it is likely not something nefarious, but rather that something is broken.