Kar is an abbreviation of 'Karabiner' which is german for carbine, and a carbine is a shorter barrel (usually heavier barrel) than its full length counter part.
So the Kar98 is a carbine.
The kar98K is a shorter version of that carbine ( the 'K' standing for kurz, which is 'short' in german )
That is my understanding as well (based on wiki

As someone suggested above, maybe Relic uses the term carabine in a different meaning, not the actual definition (??).