Yup. And just for fun, try pricing your trip with AC out of SEA vs. YVR, and you'll probably get the same affect - the price will be lower out of SEA.

Yes, it is market, and it is competition, but don't forget to take into account habit and preferred carriers. All else being equal, YVR folks are going to have a natural preference for AC, since they use them for domestic and even transborder (they have more nonstop options). In this case, if the prices are the same, the YVR pax will go to AC. Not so from SEA. Not to mention, even if there was no preference, connecting at YYZ with AC from YVR is going to be easier as you don't have to do CBP, etc. So UA lowers its prices out of YVR to counter the disadvantages so folks will fly with them instead.

It's the opposite for AC at SEA. So they often have lower prices than UA (DL, AA, etc.) since pax their have the US-carrier preference, and the extra hassle of customs, etc. when the layover is in Canada.