"Many of us are using the Zimbra UI." Are you using it exclusively? The most logical cause of this would be an IMAP client such as Thunderbird doing its own spam classification and moving things into the junk folder. . .that and users (not you obviously
) dragging messages to the junk folder using an IMAP client.
In theory either the "junk" button or dragging to the junk folder IN THE AJAX CLIENT should train the filters properly, but doing so in other programs will not.
I'd also wonder if perchance you have any other mail filters set up that might be moving messages into the junk folder. . .because if the spam engine itself were doing so, they'd have to have the "X-Spam Status: Yes" set, which your sample does not.