The Easton arrow chart is a guide for proper arrow selection. Easton even states as much right on the chart. But from everything I have seen in my 20 plus years in this sport, not many archers vary from what info the chart gives.
I have not used the arrow chart exclusively, but I have refered to it when things just were not adding up. (improper arrow flight)
As to your question of Easton going too heavy or too light in the choices they give, I would lean to the heavy side. I would also say that this is to error on the side of safety. One must remember that for years upon years, archers shot arrows and harvested game with heavy arrows. ( by todays' standards.) And with the need for speed so pronounced these days, I think that easton is smart in doing such.
Tweak, tweak, tweak...