Sumx measure result differs when replacing an expression with an identical variable

When I enter part of a sumx expression as variable instead of directly coding it in the measure, I get a different (the expected) result.

Below a simplified example. Sample data =

Expected result of measure

Multiply the balance of each row with the max selected month. So when both months selected, multiply each 500 with 2.
Note: I know in this simplified example I don't need sumx but can simply do sum(Balance)*calculate(max(allselected,Month)). But in the more complex real life example I do need a sumx measure and the difference is same as below.

I originally created this measure 'Balance x maxmonth':

var _sumx = 
    sumx(Balances,Balances[Balance]*
        calculate(
            max(Balances[Month])
            ,ALLSELECTED(Balances[Month])
            )
        )
return
_sumx

This does not return the expected result when a visual is aggregated on month level. However if I replace the 'calculate' part of the sumx expression into a variable like below it does:

Balance x maxmonth with var = 
var _maxmonth = 
    calculate(
        max(Balances[Month])
        ,ALLSELECTED(Balances[Month])
        )
var _sumx = 
    sumx(Balances,Balances[Balance]*
    _maxmonth
    )
return
_sumx

Results with and without variable for the calculate part of the expression:

Can anyone explain why the use of a var could lead to a different result outcome? 
I'm happy with the fix but want to understand this difference in behavour when using variables within a sumx expression.

Many thanks.