Jesus...
OK, soreness is not fully understood as of now, the most commonly accepted theory of muscles soreness is that macrophage activity is the cause. So, why would there be more macrophage activity in your upper chest and not your lower? Well, let's take a look at the chests anatomy (since someone brought it up). The pectorals major has two heads, the clavicular and the sternal. Clavicular = upper, sternal = lower. Then we have the pec minor which runs underneath the pec major just about directly to the side of the armpits. The pec minor IS a sperate muscle, with a seperate insertion and orgin, and a seperate function (it intializes a press). If your pec minor is sore, your upper/outer chest will be sore. If your pec major is sore, you whole chest will be sore. If both are sore, the whoe, chest will be sore, but moreso in the upper/outer portion. Now one of the pec minors functions is forward movement away the midline of the body; moving the scapula away from the spine during protration of the shoulder girdle. Yep, this is an incline press. So, an incline press may make your upper chest more sore than your lower because the pec minor has been worked to a greater degree. This is not making one part of the chest grow more than another, this is making one muscle grow more than another. The clavicular and sternal heads of the pec major cannot be seperated (these are what people refer to as the upper and lower chest), because the human anatomy does not allow it. Not even for Arnold.