As I understand Spotify et al may do something a bit more sophisticated, but the traditional model for podcast analytics purely tracks downloads, which could very well be your client auto-downloading a subscribed episode you never play. I don't think anyone actually has visibility of "listens". And the traditional model for ad sales is a creator (or an agent on their behalf) emailing a brand "Hey, we make this podcast which gets X monthly downloads, want to buy an ad read?" I think they usually point to iTunes store rankings to somewhat support these claims but again, iTunes just tracks downloads. (Obviously, this is all rife for fraud.)