"In practice whether this actually works is all over the map. Some chains will deny the claim if the cancellation terms are even slightly different, e.g. they'll say the rate on the official site was more expensive but doesn't violate the best-rate guarantee, because it allows cancellation until 6pm, instead of 5pm, which is a better set of terms. This then makes it easy to discount in third-party channels without technically violating the guarantee, by deliberately modifying the terms slightly. Other chains follow closer to the spirit of the "guarantee", but it's a bit of a lottery."

Best Buy is guilty of this too. Not their worst offense, but a lot of things that are harsh on price match, like hard drives, often have their own retailer-specific model number. The same drive might be identical, but the one on Amazon has a different model number to BBY to thwart price matching.