I would argue that with its ability and speed, it wouldn’t need to be an A rank in power. A single D or C strike would have the same, if not greater, impact as an A strike, with the benefit of being able to continue over time. 

Edit: Unless you want to go with the explanation that it gets that A rank specifically because of the ability. Sometimes stand stats take ability into account, sometimes it's just pure raw physical ability. It's inconsistent and weird that way.