Reduced Ethermine Payouts

Sorry all if this is a ridiculous question, but I've been reading the Ethermine FAQs and agreements but i cant get any info (or at least cant figure out) how payouts are calculated in general. I've noticed over the last couple of months that my payouts (as measured in Eth) are consistently getting smaller. Obviously the price of Eth has shot up so i'm wondering if the payouts to the pool are somehow tied to a dollar value at any given moment? Any insight into why the reduction (my hashrate and uptime has been pretty consistent) would be very much appreciated!