Thats the difference with naturally grown scales and plate armor. Plate armor is fairly thin and not perfectly molded to the surface it is on. It is also fairly inflexible. It dents when impacted, rather than flexing.  

Scales on the other hand, are individually very hard, and they flex somewhat. When they flex, the impact force is spread out over a larger area. In other words, you need a lot more force to actually cause any harm. But the scales are still individually very strong, so they're still resistant to focused impacts like spears/knives/swords. Also, plate armor was typically around 1.5-3mm thick. The scales on a creature like this one are easily two or three times that, especially in the more vulnerable regions. The flesh underneath the scales is also denser, more packed with muscle. and of course inside that again, are exceedingly thick bones, not prone to fractures. There are also some funny traits to the creature's cells, which means they're unusually durable.