APY is determined by the mechanism of rebase distribution. The mechanism of rebase distribution is irregular and non-linear.

To put it in details, there is a fixed time interval T, and a rebase operation will be performed at the node P between every two time intervals, that is, we will calculate the ratio of your staked ICP amount to the total staking pool once at point P, and the ICP rewards will be distributed according to this ratio. No rebase calculation and distribution will be performed during the time interval T, the time interval T will not be larger than 36 hours, that is, every two rebase distributions will not be performed over 36 hours.

Calculation:
APY = Rn / An * Ty / Tt
Rn : Rewards in time interval Tn;
An : Averaged staked amount;
Ty : One year time in seconds;
Tt : One time interval time in seconds.