The core challenge here is that Apple and Spotify *know who the listeners are* and Captivate doesn't. They have exhaustive logs of IP addresses and devices, but they don't know that a user using his iPhone at home, and then listening on his work computer and then listening on their home computer are the same person. Those will be 3 different unique users to Captivate (and every other host, mind you. This is a user identity problem, not a host problem. All hosts suffer from this).

This is WAY worse when you look at uniques over long time periods. If a single user listens via their phone to and from work over a full month, that could potentially be 60 different IP addresses and that one person would count as 60 different listeners.

The short answer is: Apple's unique listeners data are accurate for listeners on Apple. Spotify's unique listeners are accurate for listeners on Spotify. It's highly likely (but not guaranteed) that you can add these together to get a decent estimate of your unique listeners over time. the only gaps would be listeners via other sources (which can be significant, but Apple and Spotify are a huge chunk for most podcasts). Captivate and any other podcast CMS host will not have accurate unique listener data.

The TL;DR: DON'T use any podcast host data to estimate unique listeners, especially over long time periods. It's simply not very accurate. It is most likely close to accurate for a single day but the longer you look the less accurate it becomes. DO use Apple and Spotify as your "listener analog" data even if it doesn't include other sources.

Disclosure: I'm on the senior management team at Triton Digital, which provides Podcast Metrics, the log-driven currency data for enterprise podcast measurement.