AVERAGX not summarizing correctly

Hi

Issue as follows:

I am trying to calculate the difference between two dates and then average it at particular dimensions

AVERAGEX (
    ARcurrent,
    DATEDIFF ( ARCurrent[DueDate2], SELECTEDVALUE ( MTH[Month End Date] ), DAY )
)

At the Line Level it is correct and even gives the correct average in the grand total.

My problem is when I try to do this at the customer level it gives completly wrong results , when I put the lowest level of granularity in to the row of the matrix (so you can drill down to the line) at then corrects the result at the customer level.

I am completly confused with this one.

Thanks

Alexander