Governments don’t buy and sell oil, companies do. Maybe some American refinery got a better deal on Canadian oil one day and a Canadian company got better deal on American oil another day. Between individual contracts, currency fluctuations, etc. it might make sense to export in both directions for the specific players involved. Also, there are different grades of oil that are better suited to different uses. So oil sands from Canada might be more feasible to use at certain times or for certain uses, while gulf crude might be more economically feasible at other times.
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