Dragline,
After a real close inspection of all 3 guns (1 96 and 2 92's) it would appear that the main point of contact is between the front part of the slide were the recoil spring seats and the dust cover rather than the rear lower part of the barrel and the frame. It looks like when the rear of the barrel contacts the frame it may be traveling at a higher speed than the slide when it contacts the dust cover, due to the barrel having been unlocked from the slide earlier in the recoil process.
And I could be wrong here also!
I'm pretty sure the 96 barrel weighs less than than the 92 barrel and as such carries less inertia when contacting the frame which may lessen this problem. This still doesn't explain the differences in the flatness of the machining in this area.