The default values used for MonitoredItem Aggregates are the same as those used for historical Aggregates. They are defined in 18.104.22.168. For additional information on MonitoredItem AggregateFilter see OPC 10000-4.
When calculating MonitoredItem Aggregates that require the use of Bounding Values, the bounds may not be known. The calculation is done in the same manner as a historical read with the Partial Bit set. The historian may wait some amount of time (normally no more than one processing interval) before calculating the interval to allow for any latency in data collection and reduce the use of the Partial Bit.
A historical read done after data collection and the data from the MonitoredItem over the same interval may not be the same.