I believe the 70 % Readiness Rate applies to Block B aircraft from some accounts I have read. The Block A machines fall short of that number as they do not have the improvements that come standard on the Block B aircraft.
They achieve that level of Readiness by adopting the definition that the aircraft is Mission Ready so long as it can carry out just one of the multitude of possible missions.
The Army at Fort Rucker used to do the same sort of thing.....an aircraft was flyable if it could lift off the ground under its own power. A Chinook with a single engine attached to the airframe was thus deemed "flyable" for maintenance purposes and the civilian contractor met the terms of the contract for pay purposes.