I see this all the time, and it's laughable all those people selling SEO services showing they have first page positions, it doesn't mean you have traffic at all.
I have a theory that Google is testing your page for that particular keyword phrase. I rank very well for highly competitive terms, the ranking never changes but traffic has gone up consistently every month for pages who have good content and a highly relevant to the search query, in otherwise G likes your page so it will send you more traffic. For pages with crappy content but high rankings I don't see much change in traffic even though there should be more.
As for how the limit the traffic even though you have the ranking I have no idea! Perhaps hiding and showing it at different times of the day, or hiding to different groups of IPs. But having said that, I've never NOT seen my website there or when I've showed other people my rankings (who would be on a different IP range) they ALWAYS see it too. It's quite a clever thing G have done, I would love to know how they a limiting traffic out of curiousity