Airline pricing is complicated. It's even more complicated when you're dealing with multiple airlines, which have various codeshare agreements with each other. It's possible that when you buy through the United or Air Canada sites, you have to pay more because each airline is paying the other a premium rate to put you on their planes for half of your trip. Orbitz and similar sites have completely different agreements with all of the airlines. If you booked the United leg on united.com and the Air Canada leg on aircanada.com, you might pay a total rate that's closer to what you see on Orbitz.