This is such a useless guarantee...say you find a lower fare somewhere with exactly the same details and conditions. You then need to purchase your ticket at QR.com at the same time that lower fare is available on another website.
Then you're facing the big hurdle; you need to be able to contact someone at QR who knows of this price guarantee and is able and authorised to help you. You then need to prove to this person in minute detail that this other fare you saw had exactly the same terms & conditions as the higher fare you purchased on QR's website.
Then if you manage to achieve all of this, they will (undoubtably) after weeks of waiting re-imburse the fare difference. Not a penny more. Most hotel Best Rate Guarantees at least give you another 10% or a free upgrade or something a bit extra.
What's the incentive here? Why on earth would anybody go through all that trouble just to get the same fare he could have bought elsewhere in the first place?
This is just a big marketing scam to make people believe they get the best fare directly on QR's website when in reality more often than not it's not the case.