Why do I shoot enemies in the left arm most of the time?

Solution 1:

First, you are not the only one who has this. It's very-very common.

Second, it happens because the left arm, which holds the front end of the gun, also covers a (huge) part of the chest and abdomen.

On this picture you can clearly see that when an opponent faces you(and it happens most of the time), his body is greatly protected by his left arm.

P.S. Switching weapon to the left hand won't help, other player models will still have weapons in their right hands.

EDIT

As Salman A mentioned in his comment, I'll also add my profile picture:

As you can see, my left arm is also covering my body(although it's a very bad idea to face an enemy like that) :-)