The Zsync information on https://help.ubuntu.com/community/ZsyncCdImage is showing information for the build: raring. An update to saucy would better be suited.
Updating the ISO

Ubuntu archives provide .zsync files using quite similar URIs about the various flavours. For example:

zsync http://cdimage.ubuntu.com/daily-live...i386.iso.zsync

will sync the server's daily unstable raring (13.04) desktop image (for i386) to your local system with an older desktop image already stored on your hard drive.

Note 1: If you are using other flavours of Ubuntu don't forget to add the flavour's name in the zsync path after cdimage/, e.g.

zsync http://cdimage.ubuntu.com/kubuntu/da...i386.iso.zsync

zsync http://cdimage.ubuntu.com/lubuntu/da...i386.iso.zsync

Note 2: If you are using another architecture don't forget to change i386 in the zsync path with the correct architecture name, e.g.

zsync http://cdimage.ubuntu.com/daily-live...md64.iso.zsync