There are three main variants of the PW4000, the variant used on the B777 is only used on the B777, and the fan design and diameter is one of the major differences. I'm not sure since public information is a bit scattered and sparse, but I believe the -112 version is the only one with hollow fan blades. Since this was a fan failure, it stands to reason they would only ground engines with similar fan construction.
Bad luck for Boeing, indeed. Not their 'fault' of course, but I wouldn't say they don't deserve it anyway.