From what i remember reading from nord_shazbot in game :
The average score is calculated. (i think it's one for both team)
If your score is above the double of this average, then you get valor.
Something like that.
So it means if one team is steamrolling the other one, even the first player (score-based) of the defeated team may not get valor, though 5 people from the winning team can.
I admit it's kind of silly, and don't really like the idea. There must be one out of five times when a good player who deserves it is on the loosing team, did his best, and got the valour. But the four other times, it's just the winners getting it.