Custom Measure is not providing correct value as per the formula

Summary:

Created one custom measure to calculate the shipments history average using the formula as - Safe_Division(Moving_Total(Shipments History 1 Year Ago, 12), 12, NULL). As per the function of Moving_Total in this formula Custom measure should calculate by considering next 12 months shipments history 1 year ago values. But this is changing as we change the time period in the table.

Ex: if we select 18 months in the member of table, the value of custom measure will change that will be computed by considering 18 months shipments history values, if we select 13 months, it will calculate using 13 months data, if we select 6 months, it will calculate using 6 months data then what is use of mentioning 12 in the Moving_total function.