Why ArPen is bad after 50.32% with Grim Toll

Why ArPen is bad after 50.32% with Grim Toll

A player with 50.32% ArPen - SEP of ArPen until this point is up to ~1.10
| - 10 seconds - || - - - - - - - - - - - - - - - - - 45 seconds - - - - - - - - - - - - - - - - - |
Grim Toll proc 
50.32% ArPen

A player with 70.32% ArPen - SEP of ArPen after 50.32% is ~0.80
| - 10 seconds - || - - - - - - - - - - - - - - - - - 45 seconds - - - - - - - - - - - - - - - - - |
Grim Toll proc
Wasted ArPen20.00% ArPen
50.32% ArPen