I think the Zone Outdoor Air Drybulb is the Site Outdoor Air Drybulb recalculated to account for the temperture difference due to atmospheric variation, which in EP is based on zone/surface centroid height[1]. Site wind speed is recalculated for the same reason.

I briefly skimmed the I/O reference, but can't find the actual term, but I'm sure it's in there somewhere.

1. https://bigladdersoftware.com/epx/docs/9-6/engineering-reference/outside-surface-heat-balance.html#atmospheric-variation