The balance in processing shows the approximate amount of tomorrow's accrual. It is a guide, not an exact figure.
How money gets to your wallet
How the balance in processing is calculated
Based on your current hashrate and yesterday's profitability: the system can't predict tomorrow's profitability. At 03:00 Moscow time, when a new settlement day starts, the calculation starts over. At 12:10 Moscow time the new profitability becomes known, and the calculation is updated. The figure changes throughout the day.
Exact amounts are only in your accruals.