[GAMEPLAY] Karthus Ult CD not being reduced by CDR despite the tooltip saying it is

I was playing Karthus earlier in a ranked game and I was level 17 with a rank 3 ultimate and 39% cdr and my ult had a cooldown of around 90 seconds but when I ulted my ult when on cd for around 150 seconds. It was not a visual bug. I actually had to wait 2 1/2 minutes for my ult to come back up when the tooltip specifically said I had 39% cdr and a rank 3 ult and the cd was around 90 seconds. In the chat log during the game, I even stated it in the chat and linked my ult cdr in chat which said it was available in 133 seconds.