I don't really know how the discount is built into the price, but it's a result of the fact that only about 10% of the oil Canada produces can make it to a port to be sold to non-US buyers.
A common point of political contention up here are various pipeline issues, precisely because it potentially permit access to more markets, improving the sale price. (However it would also raise the price of gas in Canada for the same reason, with all the implications that comes with that).