The effective extra 100 potency is not misinformation, but relative effect.

If you are using Inner Beast while Unchained is not up during Defiance it would deal the same damage as a theoretical 400 potency attack that was effected by Defiance's -25% damage penalty.

Unchained's actual effect is to negate the -25% damage penalty of Defiance for 20s while its relative effect is to increase the damage of all warrior attacks save Inner Beast, Steel Cyclone, Fellcleave and Decimate by +33% while in Defiance.

Inner Beast's actual effect is a 300 potency attack that ignores the Defiance penalty and its relative effect is that of a theoretical 400 potency attack that would be effected by the Defiance damage penalty. The math comes down to 300 * 100%/75% = 400.