Hotel Best Rate Guarantees are mostly vaporware. Hotel chains want you to book directly through them because it costs them less to make a reservation themselves than it does to pay a commission to Expedia or other online travel sites (or travel agents). So they want you to believe you’ll get the best rate booking direct.
Best rate gurantees are largely a marketing claim rather than a promise, and they generally contain so much fine print and so many hoops as to be meaningless (on purpose).