You are confusing browser cache hits with a CDN/edge server cache hit.jQuery, or MaxCDN for that matter has no idea what the "hit rate" of a browser cache is.
This sentence should be a big clue: "We usually average around 15,000 hits per second to our CDN with 99.8% of those being cache hits."
"We" in that sentence is Kris Borchers speaking collectively about the jQuery foundation, talking a MaxCDN interviewer. But he is not talking about the browser cache. He can't be, because jQuery, or MaxCDN for that matter has no idea what the "hit rate" of a browser cache is.
Example: If I go to 1.example.com, which links to maxcdn.com/jquery.js, and then later I go to site 2.example.com, which links to the same maxcdn.com/jquery.js file, my browser doesn't send any requests! That is the entire point of far-future caching! I was able to use the version of jquery that was in my browser cache. However MaxCDN, or jQuery for that matter, have no idea this hit took place.
By the same token, if I go to 1.example.com, which links to maxcdn.com/jquery.js, and then later I go to site 2.example.com, which links to a different URL like maxcdn.com/master/jquery.js, my browser has a cache miss. /master/jquery.js is not in my browser's cache, I've never been there. MaxCDN, or jQuery for that matter, have no idea that I requested something different then before.
CDN cache hit rate has nothing to do with browser caches. In fact, people that are not you, being able to detect if something is in your browser cache or not, is a massive security problem. See my talk at BlackHat in 2007, Many of Jeremiah Grossman's talks at BlackHat (2006, 2007, 2009), or go all the way back to Ed Felton's work on using the timing side channels against browser caches.
In the industry, "99.8%" cache hit on a CDN's edge server means that 99.8% of the time the edge server can handle the request, instead of the request having to go all the way to the origin. They have no way of knowing how often a random person on the internet loads a random file from their browser cache.
This whole thing proves my point: Calling shared, common, publicly hosted copies of popular JS libraries "CDN's" or "JavaScript CDNs" is just confuses people. CDNs are about reducing latency. Share JS libraries are about trying to avoid requests altogether by leveraging the browser cache, and they are largely ineffective.