According to other threads I've read, AC has set the Tango fares on transborder routes to $1 more than the Tango+ fares so that the Tango+ fare will be picked up by the travel search web sites and by TAs. Apparently AC recognizes that the Tango product is not competitive with the US carriers on those routes and wishes to compete based on the Tango+ fare class instead. I don't quite understand why they can't remove the Tango fare class from these routes altogether, but apparently they can't.