I suspect that this is related to the fact that most any UA flight out of Houston connects at one of the hubs...you have to 'snag' the cheap fare bucket on both legs to get the cheap fare...but when flying the CO nonstop you only have to 'luck out' once and find one leg with a cheap fare bucket seat. Say you're going from iah to bos. On CO you'd go non-stop mostly likely...need to find one cheap fare bucket seat...but on UA you'd go iah-ord-bos and would need to snag the cheap fare bucket on iah-ord And ord-bos...a bit tougher often. (IF I understand the whole fare system correctly...which might be debatable )