causes of inconsistent formula

"Inconsistent formulas in the region" error is simply Excel's way of flagging a function that seems to Excel different from functions nearby. Fro example, if you have identical VLOOKUP() formulas with a SUM() formula in between, Excel will flag the SUM() formula for you to allow you to check if that SUM() function is correct or if it should be a VLOOKUP() function like the other two formulas around it.

If the SUM() and VLOOKUP() formulas are correct, you can ignore Excel's warning. If the little green triangles really irritate you, you can go into Excel options and uncheck the appropriate error checking options.