Any posts that you make that don't appear at the time but do appear later without you reposting them means that for some reason your browser isn't seeing the updated page. Once a post has been made, the PPRuNe server has no physical way of sending out the earlier page as it no longer exists on the server.
There are two common reasons why you keep seeing the unchanged page. If you don't have "Check for newer versions of stored pages" set to "Every visit to the page" then you'll keep seeing old pages as your browser will just go to your local cache. Secondly, your ISP may themselves be caching pages to save bandwidth costs.