The set is masterwork full plate, nonmagical, and it grants only +7 AC, with a max DEX bonus of 1, for a total 18 AC. Unless full plate is less protective here than in standard rules, nonmagical plate should have a base AC of 8 rather than 7.
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Set Has Low AC
Re: Dire Snapping Turtle Plate Set Has Low AC
Item quality also has an affect, this is low quality armour and has a lower ac on purpose. Otherwise the npc wouldn't so easily part with it all.
A goblin, a trickster, a warrior? A nameless terrible thing, soaked in the blood of a billion galaxies. A most feared being in all the cosmos. Nothing could stop, hold, or reason with it. One day it would just drop out of the sky and tear down your world.