There are many factors that will affect staking APY:
Validator commission rate.
Validators can modify their commission rate at each 24 hours.
Validator total voting power.
For a validator, when its voting power is getting larger, the proportion of a single staker will get less. As a result, lesser rewards will be distributed to the staker.
Validator maintainer ability.
Validators with poor uptime will jailed more easily. If the validator is jailed, all stakers on the validator will get no reward.
Other validator staking changes.
Top 21 Validators might be dethroned if other validators start getting more voting power. Therefore, it is important for the staker to stake more asset, or to move staking to other validators
BCStakingProxy will monitor changes on all the below factors and dynamically calibrate staking strategy to achieve better APY.