DAL-IAH-SEA is cheaper than IAH-SEA because DAL is the home base of America's top low-fare carrier, Southwest. So, the fares at DAL tend to be cheaper. On the other hand, Southwest does not serve IAH (it serves another airport in Houston -- HOU). Therefore, Continental is matching Southwest's low fares between DAL-SEA but has higher fares on IAH-SEA.

On the SEA issue, SEA ALWAYS has higher fares during the summer. In past years, I've paid $300 r/t during the summer but only $200 r/t the rest of the year. So, with the price of fuel, the fares have become even more expensive. However, I still managed to grab a PHX-SEA r/t for $293 for the 4th of July weekend (but I did check EVERYDAY for a month).