Hi Jonathan,
thanks for pointing this out!
Due to concerns over lack of compatibility between even minor versions of the libraries we host and because we can't set good caching headers on these URLs (because the version might change anytime), we have stopped updating the "automatic" version of the libs some time ago. This means, that /jqueryui/1/ still points to version 1.10.4 (released January 2014). This is true for all other libraries we host as well.
My explanation of this behavior would thus be, that the "actual" background image you linked to is the image as it was in jQuery UI 1.10.4.
If you want to use the newest one, you'd have to refer to the exact version, as in your "expected" link.
Sorry for the confusion. I will add a note to the website explaining this change in behavior for people who still know about the old "automatic version" feature.
Hope that helps!
Cheers,
Philipp