It looks like the Ajax cache option is set to false.
I think the whole point with the callback parameter we see in the screenshot is to make each call unique, and there is little point in caching something that is always unique. (Read more about the cache option in the Ajax API documentation.)
So, essentially you're telling the browser to cache something in one place (the appcache manifest) and you're telling it to not cache that same thing in another place (the ajax request). Maybe you get the behavior you want if you remove the ambiguity by setting the Ajax cache option to true?