Ok, so I've got a basic spreadsheet that calculates a monthly return at the end of the month:

=IF(C12 = "", "", IF((MONTH(C12) <> MONTH(C13)), ((K13 - AC13) / AC13), ""))

Cell C12 is blank, but every cell below it in the array has a date in descending order (newest date goes at the top, in Cell C13). So, on a new day I'd like to copy row 13, then insert the copied cells, followed by changing the old info with the latest info in the row 13. When I do this, however, the following are what I get in column M, where I'm having the problem...

Cell 13: =IF(C11 = "", "", IF((MONTH(C11) <> MONTH(C13)), ((K13 - AC13) / AC13), ""))
Cell 14: =IF(C12 = "", "", IF((MONTH(C12) <> MONTH(C14)), ((K14 - AC14) / AC14), ""))
Cell 15: =IF(C14 = "", "", IF((MONTH(C14) <> MONTH(C15)), ((K15 - AC15) / AC15), ""))

My question, why do the cells in red above do that? ie, they should be c14, c13, c12. But they skip c13, then descend. The rest of the formula shifts properly, but not this part. I have an identical problem in a different column, but all my other formulas in other cells that shift down work perfectly. What am I missing?