If you use custom amounts and they don't add up to the exact bill total, the calculator will use the amounts you entered as-is. It won't automatically scale them to match the bill. This is intentional so you can see if there's a mismatch. You should manually adjust the custom amounts until they sum to the bill total, or switch to a different split method (like equal or percentage) that automatically ensures the total is covered. Small rounding differences (a few cents) are normal due to how currency is rounded, but larger mismatches should be corrected.