I'm not trying to start another flame-war. I realize that supposedly you can't isolate different parts of the pec, or other muscles.

My question however, is why do different exercises make different part of the pec sore? Normally I do flat and incline benching, and sometimes flyes. After these sessions, the outer and upper portions of my pec are sore, near to, but not actually at the pec-delt tie-in. On days when I do more flyes, the inner part of my pecs, near the sternum are sore. Well yesterday, I did flat, declines, and flyes, skipping inclines, and the upper portion of my pec is hardly sore. What's more, is that the lower portion of my pecs, below the nipple are sore, and they are NEVER sore after chest day. Thinking back, the only times they DO get sore is after doing declines. I'm just curious about all this, and what you guys think, and how this relates to a debate on why you can not isolate different parts of a muscle.