MAA not summing correctly

Hi all,

I'm trying to make a moving annual average (over 13 periods) of incidents, however my MAA keeps under reporting. Here is my MAA measure:

MAA Incident Count = 
IF([Count_periods_present] = 13,
    CALCULATE (DISTINCTCOUNT('AttributionEvents_v1 1 0_Eastern'[Incident Number]),
        FILTER(
            ALL('Financial Year & Period Data'),
            'Financial Year & Period Data'[Index] >= MAX('Financial Year & Period Data'[Index]) -12
            && 'Financial Year & Period Data'[Index] <= MAX('Financial Year & Period Data'[Index])
    ))/13)

It is important that the measure only counts the distinct incident numbers within each period, as there are many duplicate incident numbers (the incidents can contain several "events"). 

The table below shows an incorrect total, and this is the value (677) that the MAA is using rather than the actual total of 909:

How do I get my MAA to correctly sum the period incident counts to 909?

Thanks,

Tom