When I click the Show All (All) button, I don't actually get all the messages in a discussion.
Answer:
There are a couple of parameters setable by both the sysop in the General Services Control Panel, and by each user in their Preferences, that control the number of messages to display on a page as well as the total number of characters.
That being said, the answer is that "All" isn't really "all".
Some customers have tens or hundreds of thousands of messages in a single discussion. Displaying all of them at once would be a huge download, and perhaps even crash the browser. So there is a hard-coded limit in the server to only show 500K worth of "all."
If you truly need to display all, you'll have to write your own custom code to handle it, or contact Professional Services by emailing sales@elliptics.com for a quote on doing the work for you.