The "bit more than the DGM" is just variance and time delays between the time the DGM estimate is calculated and the actual amount paid. It'll always be slightly off.

If your estimated DGM payout is decreasing it means that part of the payment is being put off to a later round. So the bigger hasher is getting more of this rounds payment, yours decreases but you'll get it paid over a longer tail. It's a bit frustrating to see the DGM decrease, especially with the pool having such a long time between blocks, but it's just the way the payment system works out.