Issue calculating standard deviation for difference Actual and PrevMonth value

I like to calculate the standard deviation for the difference between actual and previous values. I created measures to calculate the actuel and the valiousof the previous month and for the difference between the two. I wanted to stdev.p formula to calculate the sd for the differences but notices that you can only use columns and not measures.

So I wanted to switch from measures to calculated columns but notices that the values between the two are not the same. Althoug I used the same formula?

Anabody knows how to solve this?

Tairo