As per clarification in BLE Core_v4.2 spec, "The Advertising_Interval_Min shall be less than or equal to the Advertising_Interval_Max. The Advertising_Interval_Min and Advertising_Interval_Max should not be the same value to enable the Controller to determine the best advertising interval given other activities."
Because advertising events are perturbed in time using advDelay, the actual advertising interval should not be identical with the value, which is set with the two values(max/min) same.
Could you elaborate for us what the documentation means by "...to determine the best advertising interval given other activities"? An example would help clear some things up.