I believe that overcharge works off of your 'base stat' like stamina or armor. Then you are only allowed to go so far above that with buffs or enchants before it kicks in. You could put all 49 points into stamina and not be overcharged, but any enchants or buffs would bring you to overcharge range.

I noticed this on weekend when on my DK I would hit the softcap for armor with spiked armor. I think it stopped me at 321 armor at level 8. I noticed that some of my armor was level 6 so I made level 8 armor and tried upgrading my chest to see how that worked. My armor was now over 321 and no overcharged until I turned on spiked armor.

Is this the way it works?