if you are not making any selection in other filters, then there will be no change on the result. But if have made any selection, then will affect. You can add set modifier in the expression and nullify all other selections.
if you want to show for the selected month only, so for rest of all the filters, do the nullification as below
sum({<Year=, Quarter=>} Sales)
and if you have hard-coded for a particular month, and want to have no change with any selection in any filter, then write this expression
sum({ 1 <Month={'Jan'}>} Sales)
