Karthus has been bugged for a long time, or his tooltip is bugged

https://m.youtube.com/watch?v=1VrHFsU425s "I did not make this video, nor is this video even about the big I am describing" Still, pay attention the after they press q, and the cooldown actually counting down from 2 seconds, instead of the 1 listed on his tooltip and champion page

If you use karthus q, according to your cool down bar in game, his Q cool down counts down from 2 seconds. However, his cool down tooltip states that his cooldown is 1 second. Can someone explain this? If I am incorrect about this, please correct me, but I am quite sure that his tooltip is wrong, or his in game cooldown timer is bugged. Very sorry I do not have a video or screenshot for this