Matrix Table Row subtotal different with each row

DISTINCTCOUNT is not an additive measure by nature. Rarely, it will be additive if there is no overlap between rows. 

I recommend you to read these articles:

https://www.sqlbi.com/articles/obtaining-accurate-totals-in-dax/

https://community.fabric.microsoft.com/t5/Quick-Measures-Gallery/Measure-Totals-The-Final-Word/td-p/547907

Typically, if you want to force to get distinct count in total rows also is like this ... Try and adjust the formula to your needs. Hope it helps!

ac = 

VAR c = {"a","b","c"}
var _calc = CALCULATE(DISTINCTCOUNT(sales[cust_cd]), FILTER(sales, RELATED(cust[a]) in c
               && RELATED(cust[s]) in {'T'}    && [Net Amt] >0))

--Basically, you are looping through all the values of your category, do the calculation for each value (row) and add
 
var _calc2 = Sumx( values(Table1[Year Month Category]), CALCULATE(DISTINCTCOUNT(sales[cust_cd]), FILTER(sales, RELATED(cust[a]) in c
               && RELATED(cust[s]) in {'T'}    && [Net Amt] >0)))

RETURN IF ( HasOneValue( Table1[Year Month Category]), _calc, _calc2)