I am not seeing the $639 fare at all, though perhaps it is no longer valid, though it seems that when you have such a large radius that it only returns the first 500 flights. I am not sure how it gets to those 500, though from you data it does not seem to do it by price.
Yeah, that date is gone now, Sep 4-13 is one that's still showing the $639 price.
Same result, as soon as you change the radius to 75 miles or greater, it no longer shows the YYC -> HKG price.
I was always under the assumption that it would compile a list of every airport within the specified radius, and cherry pick the lowest fare for every combination of those airports.
Maybe someone from ITASoftware can give a better idea of what method it uses. I think one of the developers is on this forum.