In a nutshell:
Points=
Damage_Player/10 (Unless it is a headshot then it is only 2 points, possibly a bug)
Damage_Horse_Yes_Rider/5 (So basically a total divisor of 50 for attacking horses with a player riding them)
Damage_Horse_No_Rider*0 (So basically you never get points for horses that are not under player control, or riderless horses)
Damage_Friendly*-2 (So if you did 20 damage to a friendly, it would penalize you by 4 points, because 20/10*-2)
You also get points for being near a friendly player scoring damage.
Damage dealt, proximity to damage dealt and hits you take on your shield (Not sure about this last one, although it's the only explanation I can think of for why I can easily double the score of the 2nd place person on siege with my 24/15 shielder STF).
Proximity bonus, shielders due to their durable/front-line nature will receive a lot more points from proximity since they don't have to be as mobile (and thus farther away at times) from the front-line fight. If you can somehow get any other class to remain in front-line combat as long, then they too will have similar points.