Wrong total on measure with IIF - excel

Question

I have a very simple set [Sales Set]:
{
[Accounts Master].[Account Code].&[XXXXX1]
, [Accounts Master].[Account Code].&[XXXXX2]
...
[Accounts Master].[Account Code].&[XXXX20]
}
I ultimately want to create a calculated measure that will get its value from measure Gross Sales when the account is in this set or from an account group I have specified (let's call it Sample Account) and from Net Sales when it is not.
So far I have used the following measure [Sales Measure] to achieve that:
IIF
(
Exists
(
[Sales Set]
,[Accounts Master].[Customer Description].CurrentMember
).Count
= 1
OR
Exists
(
[Accounts Master].[Account Group].&[Sample Account]
,[Accounts Master].[Customer Description].CurrentMember
).Count
= 1
,[Measures].[Gross Sales]
,[Measures].[Net Sales]
)
This does what I want but the total disregards anything that is not in the [Sales Set] or in the Account Group [Sample Account].
This is what happens:
Account Group **Sales Measure** Gross Sales Net Sales
XXXXX1 **20** 20 12
XXXXX2 **45** 45 35
YYYYY1 **15** 25 15
YYYYY2 **30** 40 30
__________________________________________________________
Total **65** 130 92
Everything is as it should except the Total.
In this example the total for Sales Measure should be 110.
Is there a way that I can achieve the same functionality that I have now but also having the total being calculated correctly?
EDIT: On closer inspection the results are like these:
Account Group **Sales Measure** Gross Sales Net Sales
XXXXX1 **20** 20 12
XXXXX2 **45** 45 35
YYYYY1 **15** 25 15
YYYYY2 **30** 40 30
__________________________________________________________
Total **130** 130 92
So it seems that the total of Sales Measure is always the same as the Gross Sales total.