Annual percentage yield represents the rate of return for your interest accrued. Unlike annual percentage rate (APR), APY stresses the idea of compound interest that you are earning in DeFi. To calculate your daily ROI, divide APY by 365 for the daily rate.
Since the APY is dependent on the rates provided by the underlying protocol, there could be days where the APY is significantly higher than other days.
An example of APY being so high one day could be in response to strong demand for stablecoins being borrowed to gain more exposure to ETH. This is relevant to most of the Rari integrations like Compound, Aave, and dYdX as these are all money markets. On the other hand, mStable provides its APY through the minting of their own native currency which is then deposited into the SAVE contract to maintain price peg and create efficiency within the liquidity market. The combination of platform fees and interest being earned through the SAVE contract allow for one of the highest APY's in DeFi. If this is currently yieding higher than other Rari integrations, then the majority of the pool's allocation will be placed into mStable to allow our users to take advantage of these high rates.
Each protocol that Rari Capital has integrated will provide users with a fluctuating APY. This rate is subject to change per block as the rebalancer is constantly searching for the highest net yield and will rebalance the pools to cater to the highest APY. The more protocols that Rari Capital integrates, the higher the expected APY.