Prasat Bayon faces, Siem Reap

I do not know if that it is there but I have seen different moods and styles of architecture in the faces at Bayon temple. Honestly, I never noticed this when I was visiting the temple but while writing for my post and going through the pictures, I saw the differences.
Do they reflect different time periods or different craftsmanship or did they signify any particular reason? I do not know.

Different faces and expressions:

 serene expression, peaceful smile

 peaceful smile, eyes slanting going upwards and half open

In the upper level of temple, expressions and features on face are different. The face tower stand side by side but the features are different.
On the right hand side, the eyes are closed and lips are different, whereas the face on the left has slanting eyes half open and a broad smile. Both faces have ornaments on their head and ears and a broad nose.

closed eyes, broad nose, lips curving upwards

broad nose, curve of lips and facial expression different

broad nose, curve of lips and facial expression different

eyes looking downwards, chiseled nose, heavy ornaments of head

face here is so different, broad nose, straight lips, lacks the serenity on face

slanting closed eyes, ornate jewelry, peaceful smile

Again in the above pictures both have ornaments but the smile is different and the one on right has a raised structure on its forehead where the arrow points. The structure of the face too is different. The left picture has elongated face while the picture on right has more rounded face.

the smile is different, curves downwards, forehead has a raised structure where the arrow points. The face is round and has a broad nose, half closed eyes.
The oval structure on forehead, does it depict the third eye or is supposed to be an ornament?

The above face is very different having the absence of any ornaments, lacking the typical Khmer smile. The face has a square jaw and seems to be unfinished

faces stand adjacent to each other but the features are different. I have put a bigger picture of the two

below

eyes half open slanting upwards, beautiful smile, round face

round face, closed eyes, broad nose,

very different style of lips, broad nose, half closed eyes,  head jewelry is less

broad smile and open eyes, beautiful ornaments on ears and headgear

eyes open, round face, not much head jewelry visible

eyes closed, broad nose, upwards curving smile

closed eyes, smile on lips, broad nose

Sometimes, I think it may be the angle of picture taken or is it really so? If, I go to Bayon again maybe then I shall spend more time and minutely absorb all details.