Quote Originally Posted by Zaq View Post
SA dice aren't modifiers, either. At least, not under how I read the D&D definition. They aren't bonuses to a die roll; they are themselves die rolls.
But there's nothing keeping dice rolls from being used to modify other dice rolls, so that expression in dice doesn't change the category.
bonus

A positive modifier to a die roll. In most cases, multiple bonuses from the same source or of the same type in effect on the same character or object do not stack; only the highest bonus of that type applies. Bonuses that don't have a specific type always stack with all bonuses.
Dice rolls produce positive results. When that positive modifier is applied to another roll (as sneak attack is applied to damage), it falls under the stacking rules.

Also note that sneak attack, while it's always a positive modifier, needn't be expressed only in dice. The Craven feat makes your sneak attack damage bonus have both dice (variable) and non-dice (static) components.

Perhaps you were reading "positive modifier" and for some reason thinking of it as "positive static modifier"?