OK... maybe someone out there will help my little novice mind understand how airlines price flights.

Living in the Philly area, I am now becoming a frequent US flyer and am becoming more aware of prices (but I am sure this happens on many other airlines as well).

Here is my dilemma. Say I price a flight for PHL-SFO (flight X out bound, Y returning), the price would be $XXX. Now if I price BWI-SFO connecting in PHL I've seen the price up to 40% cheaper. And that is even calculating the same segment out (e.g.- BWI-PHL-SFO flights A to PHL and then X to SFO) and then returning.

So my question herein lies... Why does this happen? One would figure that you are flying "more" so you should be paying more, even when the same flights are involved. I know that this may be a basic question, so please be gentle.