Total measure for YTD values not calculating correctly in a table

[Anonymous]

Hi all,

For each reporting period I have YTD sales volumes, which I try to "reverse" back to monthly periodic amounts.

This measure seems to work, however the Total column in a table somehow sums up all of the YTD amounts for each month together and does not seem to sum up the individually calculated amounts per period from my measure:

I am very new to PowerBI and measures and not sure as to why the total does not sum the calculated amounts per month together? 

Does anyone have an idea how to improve this measure so that the total column is also working? The measure itself works fine per month to display the monthly periodic volume instead of YTD.

[Anonymous]

Edit:

1: The "Volume Number YTD" measure is like this:

Volume number YTD = CALCULATE(sum('Fact Stock data'[Volume]);Datetable;Datetable[Date])

2: I use the "isolated per month" measure to calculate an average also. And somehow when selecting one month, the average works fine. But when not selecting any month, the average is suddenly way lower, probably because of the same issue where the total volumes for the average are much higher than they should be.