Both, each epoch the block selection for all pools is from a random lottery based on delegation not fixed this and their performance sets the reward, however at the same time each epoch the rewards are reduced from the previous epochs rewards payout. So everyone earns a little less on rewards each epoch until the transactions on the network are enough to maintain the rewards distribution.
Both, each epoch the block selection for all pools is from a random lottery based on delegation not fixed this and their performance sets the reward, however at the same time each epoch the rewards are reduced from the previous epochs rewards payout. So everyone earns a little less on rewards each epoch until the transactions on the network are enough to maintain the rewards distribution.