Tags are fetched when you start editing a post (when ever you reload the screen, e.g. saving a draft) and then stored locally, so that working with tags is much faster. You will notice that you don't see the usual spinning wheel that appears when you add tags through the default WordPress tag box. Particularly on slow networks you often have to wait several seconds until you can continue. If your network connection causes even a small delay, the Meta Box would not be able to know whether a tag already exists - for example if your settings prevent adding new tags on the post screen.
In order to work with an up-to-date pool of tags, however, this locally stored list of tags is being refreshed every five minutes. We assume that usually tags are not edited that often, so that 5 minutes is a reasonably good time to make sure you don't miss the tag that you need.