Negative Pressure Differential Requirement

Dear all, One of the credit requirements is that: "The pressure differential with the surrounding spaces must be at least 5 Pa on average and 1 Pa at a minimum when the doors to the rooms are closed." This was deemed a bit vague by the mechanical engineer: He claimed that if the exhaust system is designed for a differential of at least 5 Pa, then why worry about the 1 Pa? Can someone please clarify the distinction between both numbers, and what's the proper way of designing the system to achieve the requirement? Many thanks.