Why is the version number of a package in Ubuntu repositories so different from its original one?

In synaptic I saw okular 4:4.13.3-0ubuntu0.1, and on okular website, the latest stable one is Okular 0.21.

Why is the version of the Ubuntu one so different?

How can I find out the actual version of okular in the Ubuntu one?

my Ubuntu is 64-bit 14.04.

Thanks.