I think google has bots which usually crawl a number of pages at a time -- whereas yahoo seems to crawl one or two pages at a time. As a result, if google and yahoo only grab a robots.txt per 'session', yahoo will pull robots.txt many more times because they are getting fewer pages at a time.
So, google may have hit you with only 22 different sessions that each grabbed 17 pages and yahoo may have hit you with 235 sessions that grabbed one or two pages at a time.
You might also look at the number of Unique IPs that google sent versus the Unique IPs that yahoo sent. Perhaps more machines from yahoo hit your site.
As for the number of pages, who knows how those engines decide what to crawl. Google may have already cached your page structure and Yahoo may be still discovering pieces of it. Hard to say really.
All conjecture and guesses.
