[Gameplay] Ultimate cooldown is longer than the description

After casting Karthus ultimate it went on cooldown for longer than the cooldown time described. It occurred while I was casting ultimate without being in Karthus passive but I'm not sure what other variables there might be that triggers this bug. I have noticed this bug in both Summoner's Rift as well as ARAM. In the link I've also included the rune page used as I think it's possible that it was a reaction to a fully stacked Ultimate Hunter.