Airline Logic

Sometimes I really do not understand airlines. My parents have been planning on hosting a picnic next weekend which Brandy and I were coming up for. I was going to drive, but in the interest of not making an 11 hour drive 4 times in 3 weeks I decided a flight may be better. So, we search for flights. The best we can find to BWI was over 400, which I thought was a little excessive, but I really didn't want to drive again, so the level at which I'll accept a flight price has in fact increased on this one. I went ahead and checked the 'search nearby airports' because I figured perhaps DC flights are a bit cheaper - sometimes they are, sometimes they aren't, you never know. As it turned out DC flights were not cheaper but a Harrisburg flight was, only 300! I looked at the flight and realized it was flying through Dulles, so I searched that specific airline just to Dulles (a longer drive, but if the flight is cheaper it may be worth it)...that flight was 600. Now really? Why is it cheaper to get back on a plane and fly to a new destination than to end my destination there? My dad said maybe I should check if it's only 200 to fly to Denver and then back to Harrisburg :)