Hm. Their site says the same thing. Perhaps weapon weight affects the probability of chance in the calculation. Like, it's easier to get lucky kills with different weaponry. Bastards.
Quote: Originally Posted by http://www.psychostats.com/wiki/FAQ#How_is_skill_calculated.3F
Stats Display
[edit] How is skill calculated?
PsychoStats version 3+ continues to use the ELO formula to calculate skill values for players. In a nutshell, ELO tries to base how many points you gain or receive for kills and deaths based on your skill compared to your victim or killer. It calculates a probability of chance when someone kills another player.
Also, PS3 is currently experimenting with the ELO formula and has tweaked it a little to allow for total online time of a player to have an affect of how much the probability changes on each kill. The more online time you have the less your skill will go up/down.