it's a bug with the tooltip and not the ability.
unless you honestly think crushing darkness should hit for over 8k when it crits (for the first tick not the DoT)
I play in 1440P, that's not 4k but the difference between 1440p and 1080p is pretty much just a smaller UI and it allows more of the game to be displayed instead of UI.
http://puu.sh/64xDA.jpg