Why is the actual mining revenue very different from the theoretical revenue?

The daily theoretical value is for reference only. It is a theoretical value calculated based on past or current data. It is an instantaneous value. When the currency mining difficulty fluctuates greatly, or the mining difficulty is adjusted quickly (such as ZEC, ZEN, XVG, etc.), using the theoretical value at a certain moment, it is often impossible to accurately predict the daily revenue or produce large deviations.