well, that is a common habit so to speak, to frown. maybe they want to look more manly, more violent XD like metal bands or what...
Kai looks for example bad on photos and good when moving...and adorable cute when smiling..
Uruha..well Uruha looks funny when laughing and cute when smiling...
Reita- Reita has always the same expression lol
Ruki- does that guy smile at all??Maybe because he is so small he has to frown in order to look more serious XD
And Aoi. hmmm.....Aoi doesn´t smile often, does he? He usually looks very serious