That is pretty interesting to see that everybody get crawled at such a different rate. For me MSNBOT is #1 followed by Yahoo and then Google.
It all depends on your pagerank for google. If you have a high pagerank such as 6, google crawl your site everyday. But if your pagerank is 1 or zero, you're lucky if it's crawl once / week or even month. Another factor is how often your content changes. If you change your content often, google learns this and will crawl your site more frequently.
more the inward links, more will be the frequency. Like other search engines MSN also crawls through links.