It looks like you have set both the min and max OA flow to 153 cfm, causing a constant OA flow whenever fresh air is needed. Since you have a CV fan in the air loop, the air loop will always supply air at the same flow rate (whether it's autosized or hard-sized). So, constant OA flow and constant supply flow will yield constant OA fraction. If you were to hard-size the supply flow rate, there's then that would just alter the OA fraction to a chance that EnergyPlus is calculating a different value than the 40% you're currently seeing when autosizing the supply flow less than your hard-sized OA flow. Check the Outdoor Air Summary report and the HVAC Sizing Summary report to see if this is the case.rate.