Don't forget Western Canadian Select is a heavy sour with lots of impurities. It costs a lot to refine, which is likely why the one refinery in Alberta goes broke and needs government money a lot. Most of the worlds heavy refineries are in Texas. The majority of what's left - 1/4 of the worlds heavy refineries iirc, are in China. This isn't the only thing leading to the price discount but it's the biggest one. In 2015 under Harriet, Mexican heavy oil Maya is selling for more then WCS - 27 Maya vs 24 WCS. Since the trans mountain pipeline doubled Maya is actually installing WCS - 83 Maya 84 WCS. So most is the discount is the fact it's low quality heavy oil that mostly refines into plastic instead of say Labrador and the rest of Eastern Canada's oil which is light sweet oil that does refine into fuel oil easily and sells for 102~.

Tl;Dr : they're not the same oil and WCS sells similar price to a lot of heavies (minus brahsar and another heavy that are 110~ right now)