Hey guys i would like someone to explain to me how the weapon accuracy system works on call of duty black ops?

e.g i use the commando and my weapon accuracy was 11.97 two days ago but ever since then it has been declining,now it's 11.88.i have an average 3.00 ratio after every game.all my kills are with this weapon but it's still going down .i just dnt understand it.its very frustrating coz i always have more kills than deaths.how does this system work,i need help plz!!