While the overall effect is that they look like dragons, for actually attempting to 'explain' Au Ra physiology I'd rather point to armoured mammals such as armadillos and pangolins. (Granted, they're still fully-armoured rather than patchy, but it seems like a closer reference.) Horns and antlers are also a mammalian trait.
I wouldn't put too much weight on them "really" being able to add/remove scales at will though. It goes in the same category as being able to remove scars and alter limbal rings: on a case-by-case basis they probably shouldn't be possible, but on other races that same switch might only control makeup and earrings - so everyone is equally given access to turn it on and off.
As for how they developed "armoured" sections in the first place... evolutionarily speaking, they're going to develop where they're needed for protection - or rather, any randomly-evolved scale placements that contribute to their survival are likely to be passed down to the next generation.
Given we're dealing with horns as well as scales (the far bigger evolutionary question, really), developing armour around the face is actually probably a valuable defence against accidentally being gored by your partner... as is having the males and females be different heights so they're not going to poke each other in the eye.
It's the closest thing I've got to an explanation for the height differences, anyway...
The other body patches seem like they could be protective without getting in the way of movement.
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I also wonder if scales look more out-of-place depending on the character's appearance? For my alt, I tried to tone his hair colour in with his scales (on the logic that they're all the same base substance and might take on a similar colour) and I think that makes sense as a character design, and certainly with NPCs I think they can look mismatched when they have a hair colour that is nowhere near their scales.