Why System calculate MAPE does not match with manually calculated MAPE in case of daily buckets ?

Summary: Loaded history of 18th months and calculated MAPE manually in excel based on MAPE formula i.e., (Sum of ABS(Actual-Forecast)/Sum of Actual). Manually calculated MAPE match in case demand plan runs in monthly bucket but not in case demand plan runs in daily bucket.

Content (please ensure you mask any confidential information):

Item = XXXX01012,

MetricsPeriod = 182 Days,

Loaded history for last 18th months

Case 1: Run demand plan with monthly buckets for 12 months.

Result: System calculated MAPE is 0.06 and manually calculate is also 0.06

Case 2: Run demand plan with daily buckets for 365 days.

Results: System Calculated MAPE is 0.061 but manually calculated MAPE is .491