Empty promises. As recent as May 11 I still got email from them claiming "Low Fare Guarantee":
B747SP, we appreciate that you chose United® for your recent trip to Chicago. If you book your next flight at united.com, you'll have access to:
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A low fare guarantee – Find the lowest fare at united.com and you'll never pay a service fee for booking online.
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Easy and convenient travel planning – Book your entire trip at united.com, including flight, car, and hotel reservations. Economy Plus® seating is also available for purchase on united.com during your flight purchase.
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FareLockSM – Hold your reserved fare for up to seven days while you finalize your travel plans, then purchase your itinerary once your plans are confirmed. Your price is guaranteed while on hold.
A few years ago if you found a lower fare after booking, you could claim the guarantee and get the difference as a voucher. Now this guarantee is gone and you need to pay a change fee for that, and the fee is naturally more than the fare difference. There is a thread about a reduced fee if the booking is within 30 days though.