i believe you can work different parts of the muscle. but the reason you get different results on you tri's is because the tri is made up of 3 parts. just like your bicep is made up of two, and your quads are made up of four. hence the names. however your chest is made up of two different muscles(pectoralis minor, and pectoralis major). the major lies on top of the minor. but since the pectoralis muscles are wider than most other muscles then one side of the muscle will contract more than the other depending on the angle.