Christmas in TX

I have decided to come home for Christmas break, so recently I have been looking at flights from GUA to DFW. Today, however, the principal at IAS told me that it is cheaper to fly from GUA to Washington D.C. Curious if this was true, I compared the flights. Get this...it is OVER $200 CHEAPER to fly from GUA to DFW to IAD than it is to fly from GUA to DFW! However, they are the exact same flights! The only difference is that those continuing to D.C. have to make the connecting flight. Where is the logic in this? You would think that it would be cheaper to fly from GUA to DFW! Instead, it's cheaper to take another flight...this so weird to me!