For example, when users deposit 1000 USDC, they might receive 999 principal USDC and 1000 yield USDC. This will happen when the interest between pools starts at the moment that the user deposit is already 0.1%. At this point, we are sure that the 1000 yield token can redeem for 1 USDC. Adding up with 999 principle USDC which can be redeemed for 999 USDC, this adds up to 1000 USDC. If we don't do this reduction, then the yield token will not be fungible.