"that's how it works so that's how it works and common sense be damned"
Basicaly ... yes.
But also nope, since there is a logic if you think about it.
Armor in DnD dont diminish damage, it either protects you perfectly ... or not at all.
Therefore, if you have scaled, or otherwise hardened, skin (several races have), shell, or whatever is wraping your body providing some defense ... its just numeric value of that defence.
Your scales are tough to AC 13 ...
USUALY ... best AC calculation is used, so if you wield any armor that provides less than 13AC, then you still have 13AC (not really sure if there is any in BG-3, but this is how it works in tabletop) ...
i mean pure hypoteticaly you can pick worse AC if you really demand so, but that would only work as purposefull effort to break this explanation

but i digress ...
So, if you wear lets say 12 AC armor, but your scales provides you 13 AC armor ... you still have 13 AC armor, bcs whatever hits you have to get trough your scales to hurt you ... and whatever gets trough your scales, will automaticly get trough your 12AC armor.

Same it goes on other way ...
If you pick Lae'zel armor (for example) you get 15AC ... that alone is 2AC stronger than your scales ... so if something manage to pierce/slash/whatever trough that ... your scales are still there, but they do not represent an obstacle.

OR in simple form:
16+ Attack gets trough 15AC armor ... aswell as 13 AC armor > therefore scales are not relevant
15- Attack is stopped by 15AC armor ... so it never gets to 13 AC amor > therefore scales are not relevant
The only way they would be relevant would be if you would need to roll hit for each layer of defensive wraping people have ... and that would be OP as fuck. :-/