Karthus one for all abstaining glitch

My friends and I played two game of all for one and we abstained. The first game we got rengar so we knew it was random. We did it again and we got karthus for some reason. I think this is because rito programmed it so that one can not click on karthus to play it. Instead of removing karthus from all for one. So if you do abstain, there is a chance that u will get karthus.