THANK YOU FOR THE ANSWER b.anderson
There are other differences regarding the SV's HDZW system.
First, these SV x-rays are done on mostly 12-month-old dogs. In many cases (such as mentioned in an improvement from fast normal to OFA "good") that does not indicate a great inconsistency in the 2 systems. The fact is that a rapidly growing, immature dog x-rayed at a year (and receiving a fast normal rating) can indeed settle in with the greater maturity that comes with OFA x-rays (and be rated as OFA "good") at 24 months or later.
However, something is definitely fishy when one sees an NZ-rated dog (imported to the USA and not known for producing good hips) receive and OFA rating of "excellent".
One must also understand that the SV's system is often skewed and weighted for German dogs against "auslander" dogs by assigning the latter dogs higher ZW scores.
OTH, I have a male (now 11 years old) with a ZW score of 77. When bred to a female with a ZW score of 76, he has produced a daughter (yet to be bred) with an HDZW score of 73. I don't know how that is possible.