My understanding is that demand for points redemption is the basic driver for category rankings (recent changes notwithstanding). Demand for rewards stays in Central London is strong, so that would explain why Marble Arch is one of the properties that went up from 6 to 7. Room rates, "quality" or condition of the properties have little, if any, bearing on the designation....at least from what I've been told.
"Up the street", Grosvenor House also goes up from 6 to 7, but this would be explained by the approaching conclusion of a major remodeling project and reflagging as a JW Marriott.