There has been some discussion around the net about Google traffic throttling and the idea of Google balancing out traffic to a site based on previous traffic patterns.
I've seen at least one site sitting firmly over a year with the same level of traffic, rankings go up and down and the amount of traffic from Google evens out over say a week or a month. No matter how many links and how much content is produced, the volume stays the same.
I think they somehow set a range of traffic and adjust rankings based on how much is delivered. Having a flat line traffic graph with lots of quality being added to the site on a daily basis and a natural link growth, doesn't add up.