Well the problem with your expectation is that you are assuming an incoming data stream that has blocks of data with identical complexity. So that variation of the difficulty factor is the determinator of your payout.
This is not true. The blocks of input have a varying number of transactions and fairly widely varying coin amounts. I would expect the earnings from a block to scale more directly to the sum of coin needing confirmations. The effect of the difficulty factor is more of a speed (time) throttle.