Infiltration Rates Not Responding As Expected

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I have setup OS:SpaceInfiltration:DesignFlowRate in a model as follows:

Schedule = Fractional (Value=1 for all times) Design Flow Rate Calculation Method = Flow/ExteriorWallArea Flow per Exterior Surface Area = 0.0025 Constant Term Coefficient = 1 Other 3 Coefficients = 0

My understanding (after reading the Input/Output Reference) is that this should result in a constant flow of infiltration air (for all times) at the volume flow rate specified. However, when I review results, the infiltration rates in the model appear to vary significantly. Any ideas about what is going on?