It's like the draconic sorcerer getting a base AC of 13 because of the scales but the dragonborn doesn't.
There's nothing inherently wrong with that.
Not all scales are equally hard. Even if both are of draconic descent, it doesn't necessarily mean that Dragonborn have hard scales to the point of having that inherent +3 AC.
While the mechanic is specifically suggesting that Draconic Sorcerers do in fact gain scales that are hard enough to warrant that extra AC.
