Proportional payments vary due to the erratic nature of finding work. For steady payouts choose PPS.
The only variable is time to solve a block, with steady hash rate of my equipment and steady hash rate of the rest of the pool, my proportional part of the pool is the same, nevermind if block lasts 12 minutes or 12 hours. The longer the block is, smaller is the variance between my strength compared to strength of the pool, and that block was pretty long - 20m shares, so it has to be something else...