I understand all the settings you've mentioned, but how are you listening to these Podcasts?
It's possible to listen to a podcast "as a stream" in the Podcast app (in which case, it's not "a Podcast"). If you stream episodes as opposed to downloading them first, that will use your data allowance, regardless of your settings.
The concept of a Podcast is that your Podcast app downloads episodes in the background while you are on Wi-Fi, and then you listen to those downloaded episodes later. Once downloaded, each episode is stored on your device and you listen to that stored data. (The Podcast app can be set to automatically delete episodes after you have played them, thereby managing your storage space.)
But, if you simply click on the episode while browsing the iTunes Store, it will play from the internet, not from a stored file on your device. Yes, the device is "playing" it, but the actual file is on the internet and your data allowance is being used to stream that file from the internet to your phone, while you listen to it.
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To put this another way:
In the Podcast app, you subscribe to one or more Podcast series. If you set your phone to Only download on Wi-Fi, then while you are not connected to the internet by Wi-Fi, nothing will be automatically downloaded. But when your phone sees a Wi-Fi to which it has access (at home for example) the app will check to see if any of your subscriptions have new episodes. If they have, then the app will download those episodes via the Wi-Fi without you doing anything (i.e. automatically). They are then ready for you to listen to.
You the leave home again and while away from your Wi-Fi, you listen to those downloaded episodes, played by the Podcast app on the phone but from the stored area on the phone. Therefore, no data usage.
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There's an important distinction between streaming and Podcasting, which has not only been widely misunderstood in the past, but continues to gather pace as everyone, including professionals who should know better, get this wrong.
The episodes you listen to are audio programmes, whether you listen to them as a stream, a download or as a Podcast. But the crucial thing is how you get those programmes.
- if you listen to the audio as it is being played from the internet, that is a stream (this is also known as streaming). If you do this while not connected to a Wi-Fi signal, it is using data from your phone's allowance
- if you use your browser and go to the programme's website (or any website) and click on a download option, which downloads the episode to your device (Android phone, desktop computer, Android tablet etc.), you then use an audio player to play that stored file. (I'm not sure this is possible on an iPhone, I have an iPod, not phone). Once downloaded, these files would be played by your audio player, not by the Podcast app. If you download the episode while not connected to your Wi-Fi, then again, it is using your data allowance
- if you use a Podcatcher (an app designed to act as a Podcast downloader and player), and let the app download episodes for you to then play later (even when offline) - only then are you are listening to a Podcast. Providing you set your Podcast app* to only download on Wi-Fi, that will not use your data allowance
- if you use the Podcast app, but listen to a stream (as discussed above), then you are listening to "a stream", not a Podcast. If you do this while not on Wi-Fi, it uses your data allowance
Is it possible therefore, that you have been streaming the audio rather than downloading it?
* edited by author after initial posting, for clarity.
