I believe that the 4.13.3 part of version number does refer to version of KDE applications and not the version of okular. In July 2014 KDE updates applications to version 4.13.13 and as you can see on the download page okular package has version 4.13.3.
As from the okular website :
The last stable release is Okular 0.21, shipped as part of the KDE Applications 14.12 release.
the okular 0.21 was shipped with KDE version that is not currently in repository for 14.04 Trusty.
You can find the version of okular offered in rep. by looking at Okular website's news and finding which okular version was shipped with KDE version you see in Ubuntu reps.