on PoS you earn a specific ammount of coins which is calculated by your coin Age
*example*
11.11111111 Coins are full confirmed since 48 Hours , so the CoinAge would be result as 22.22222222
and the Anual Interest.
using DPOS calculate your CoinAge like above, but this is just used for your Coin Weight,
which is compared to the NetWeight *all People, and matured coins*
if your CoinWeight,..... *which is based based on how old a specific ammount from your coins is*
is higher then the Network Weight
then you find a PoS block and earn the reward.
BUT, the reward for that block is a fixed Value, similar to PoW blocks.
i hope i help you a bit ;-)