Why are the rewards of Dogecoin getting lower and lower?

Dogecoin is merged mined with Litecoin. f2pool currently adopts the PPLNS payout scheme to calculate Dogecoin rewards separately. Dogecoin rewards mainly depend on the difficulty of Doge mining and the block production of the mining pool.

Dogecoin’s network difficulty is adjusted frequently. As the difficulty of Dogecoin continuously rises, it may lead to a decline of Dogecoin rewards. Nevertheless, there is no direct relationship between the mining rewards of Dogecoin and those of Litecoin, and there is no fixed reward ratio.