Originally posted by DragonLord It's likely due to the limited resolution of the metering sensor (360x240). The amount of light available also affects the effectiveness of face detection.
The resolution of the K-3 imaging sensor may not be as great a limitation as you suppose.
The Nikon D610 has a mere 2016 pixel (2k) metering sensor, and yet in PDAF can still detect faces. (Proof of that is easy to see with the D610, since it can immediately highlight detected faces upon image playback on the rear-screen).
Detecting eyes and assigning them priority as focus targets is harder, but probably doable with the rich information coming in from the K-3's RGB sensor, due to it's very high resolution and -3EV sensitivity. Face detection algorithms are pretty smart nowadays.