What is APY
APY stands for annualized percentage of yield. It measures the actual reward ratio on your principal by considering the impact of compound interest. Your staked BHO are your principal; in addition, due to the rebase mechanism, compound interest is regularly added at each epoch (about 8 hours).
$APY = (1 + rewardYield)^{365*3}$
Rebase occurs 3 times a day; there are 365 days in a year; the rebase frequency will be 365 * 3 = 1095.