Karthus ult and maybe others dont show correct times for teammates with presence of mind

I was karthus with presence of mind and i killed someone with my ult, then a bit later when i had it my i told my premades that i had ult up and they said "your ult isnt up look in on the champion bar" and i said no look its up, and surprise surprise the timer was behind because my ult had been reduced by presence of mind and it hadnt caught up.