[Bug] Karthus Ultimate Cooldown

So, I just played an Aram and I noticed something odd at the end. I had 40% CDR and I was level 18. The tooltip for my ultimate said it was a 96 second cooldown. I spawned, and mashed R to get a kill. When the channel was finished, my ultimate was put on a 2+ minute cooldown. It happened several other times, where the tooltip didn't match the actual timer, during the game, but this was the most noticeable. http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/1899397661/37203769