That 's strange. I opened both URLs again and now I can see what you tell me. Earlier the behaviour has been different and I could see episode 14 after opening both URLs.
This behaviour is most likely related to the WP Super Cache plugin which you use.
In the source code of the /?feed=podcast version I can read this comment:
<!-- Dynamic page generated in 1.049 seconds. -->
<!-- File not cached! Super Cache Couldn't write to: wp-content/cache/wp-cache-e92e0fb1e440fbff823b84c41e8335fc.html -->
in the source code which I get after opening the /feed/podcast URL the comment of the plugin is the following:
<!-- Dynamic page generated in 1.645 seconds. -->
<!-- Cached page generated by WP-Super-Cache on 2012-11-14 15:35:12 -->
It seems to me that Cache plugin has trouble handling the /?feed=podcast feed URL and can not cache the content. That is why you can see the episode 14. If you open the other URL the Cache plugin shows one a version of the feed which is still in the cache memory stored.
I'm not sure why this problem occurs. But I guess you should try clear the cache memory of the plugin in order to update the feed and the iTunes Store page. (Please note that the iTunes page updates only once every 24 hours.)
(I'm going to add wp-super-cache as a tag to this thread.)