This is an ancient diablo mechanic. All oskills are capped at +3 for classes who “own” the skill, this cap was made to help balance the game. Since you are a sorc, and thunderstone is a sorc skill, it's inherently capped.
In CLoD, barbs wielding Call to Arms are capped at 3 to battle orders etc even though the runeword can roll +5. Barbs will opt to use rare BO weapons so they dont have to invest in multiple CtAs.