Perhaps the default is being set to be efficient with /%year%/%monthnum%/%day%/%postname%/ for cache plugins such as WP Super Cache that WP offers right next to JetPack as a top 5 add-on at installation. WP Super Cache can be set to ignore certain pages; "Add here strings (not a filename) that forces a page not to be cached. For example, if your URLs include year and you dont want to cache last year posts, it’s enough to specify the year, i.e. ’/2004/’. WP-Cache will search if that string is part of the URI and if so, it will not cache that page..." i.e. wp-.*\.php or index\.php