Yes - I'm curious too - I just ordered factory OEM hubs from the UK so my 88 can use these S4s wheels with 58mm center. Didn't want to bore the wheels out (to maintain the mechanical integrity) and didn't want to machine the 60 mm hubs.
It does seems odd to remove 2mm - so minor. And it's only the front - not the rear (remains at 60mm).
Maybe the smaller bore size on the wheel made them stronger? Just grasping for straws.