The diagnostic page doesn't appear to always be strictly accurate. For instance, it says "Google has not visited this site within the past 90 days." for many of my sites which it has crawled daily for years.
I assume there are different levels of data collected for different types of visits. For example, Google may just collect data for the PageRank algorithm (i.e. your pages have been visited by google.com) or they may also collect safebrowsing/diagnostic data (i.e. your pages have been visited by google.com andhttp://www.google.com/safebrowsing/diagnostic).
