problem with absolute difference calculation

Hi Landraille, what you need to remember is that in the most cases when you write a formula in PowerBI, you refer to a column, not a row. 

Let's consider your measure:

Difference abs sum = ABS( SUM('Table'[Quantity_Soft1]) - SUM('Table'[Quantity_Soft2]) )

In this case you first sum the whole column "Quantity_Soft1" then sum "Quantity_Soft2", after that you deduct them and get an absolute value as a result. It looks correct, when you have only 1 row for each combination of Stock and Item (except Total row). When you arrive to the total, the same happens: PBI first get a sum of column 1 (considering the sign, so some values cancel each other), then of the column 2, deducts them and only then finds an absolute value.

Now the second one:

Difference abs sumx = 
   SUMX( 
      'Table',
      ABS( 'Table'[Quantity_Soft1] - 'Table'[Quantity_Soft2] )
   )

Here the sequence is the following: first we calculate a difference between value in Quantity_Soft1 and Quantity_Soft2 at row level (because you used a formula with X at the end). Then, you obtain the absolute value of the deducation (still at row level) and sum all of them (so only positive numbers are summed).

With this apporach, you would achieve the correct evalutation if Stock and Item would be the lowest level details in your table, which I expect is not (maybe you have date or anything else in the same table). Remember that deducation happens for every row of table "Table", which is filtered for, let's say "Stock2" and "Item1". Are you sure this table contains a signle row? If 10 and 6 are sum of more rows, the measure doesn't consider it: it performs operation row by row.

Here is the example when Stock and Item would be the only attrbutes of "Table" using the exact same measure.

You might consider use of Summarize() fucntion before applying the measure.

In case you need more help, please send a sample of your data which includes all columns of "Table" and the details of expected result (you can do it in Excel, for instance).

I hope my answer brings you some clarity 🙂 Enjoy your day!