Your miner is trying to solve a block.  Every other miner is also trying to solve the same block.  When somebody succeeds (which can take anything from a few seconds to half an hour or more, but is on average every ten minutes) they are credited with finding that block, and everybody starts to work on the next.  Miners who are members of a pool such as bitcoin.cz each send partial results (shares) to the pool, and if a pool member happens to solve the block, then all pool members receive a proportion of the value of the block according to how much effort they contributed, and when.

If, however, the block was solved by somebody who was not a member of that pool, then nobody in the pool gets anything for it.

You will notice that the first column in your stats shows an incrementing number under the heading # : this is the round number, and counts the number of blocks the pool has solved.  The round is the interval between blocks solved by the pool, and the time in the Duration column is the length of that round.  Further along is Block # : this is the actual number of the block which was solved by a pool member; note that they are not usually consecutive, because the blocks in between were solved by somebody else.  In your stats, round 21550 ended when block 282631 was solved.  The next round, 21551, ended when block 282635 was solved some 56 minutes later.  The intervening blocks 282632-282634 were solved by miners who were not part of this pool. 

In the short term, you can expect your BTC reward to be roughly the same for each round, no matter how long it lasts, because however many shares were actually credited, your proportion of the total effort will be similar.  In the long term it will slowly decrease as more miners join the pool and your contribution gets diluted.


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