CRA Notice of Assessment says TOTAL PAYABLE was changed because of a calculation error

Hi

My NOA stated: "We changed your total payable from $x to $y because of a calculation error."

I compared all the other line number values on the NOA with my detailed tax form. All the numbers were identical except  line 42000 Net Federal Tax and line 43500 Total Repayable. 

Any suggestions on how I can dig further into this and determine why the difference?

Thanks.