Impression Measurement for each order in your campaigns is reported according to the IABv2 guidelines here. This method counts an impression when 100% of the ad file has been downloaded. Therefore, it is normal to see a discrepancy between your impressions (measuring the ad units downloads) and your episode downloads (measuring if 1 minute of audio was downloaded by a unique listener). 

Additionally, you may see that - for a given episode - your prerolls have more impressions than your midrolls, which will have more impressions than your postrolls. This is due to certain progressively downloaded listening behaviors where a listener may hear the first portion of a show, then stop the audio before downloading a later ad unit, such as the midroll or postroll.