I had asked OP:
They did not reply so I just put any dates into Google Flights. The problem is, the prices are all over the place as there are multiple daily flights.
EDIT: OK, So I re-entered Feb 10-17. Results for both AC and UA are identical flight times
Below is the UA trip summary screen shot with Fare, Taxes, Total - USD
Below that is the AC screen shot
The taxes and fees are almost identical, the base fares are not and maybe others can determine if the difference is because the the AC fares class on the outbound is Tango S versus L on the return, whereas UA base fares are lower.
Funny thing, the AC flights are operated by UA.
Now with this info, perhaps you can figure it out. Thanks.
