You're all misunderstanding the "lowest fare" guarantee United.com offers. United.com doesn't guarantee the cheapest price out of any airline, just on their own flights. As well, the ticket must be the *EXACT* same on the other site you found the fare, including fare bucket and rules. Oftentimes, Orbitz/Travelocity/etc flights will book in "Q" class while United.com may book into "S" or "T" class. This difference invalidates the guarantee. Straight from United.com:
http://www.united.com/page/article/0.../01/16&time=20
The guarantee is valid only for published retail fares available on the Internet for the exact same flight(s), travel date(s), and travel itinerary for the same number of passenger(s). Fares must be in the exact same cabin(s) (First, Business, Economy) and carry the exact same fare rules and restrictions.
The Low Fare Guarantee does not apply to unpublished fares that are not available to the general public and fares that cannot be purchased on united.com or flyted.com. Fares not available to the general public include, but are not limited to, corporate, military, government, wholesaler, consolidator, opaque agencies and other discount fares. The Low Fare Guarantee does not apply to package fares, such as airfares sold as part of a travel package, nor does it apply to airfares on another website that have been reduced as a result of promotional discounts, such as dollars off coupons, loyalty program discounts or fly free offers. The Low Fare Guarantee does not apply to tickets that you choose to refund.
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