I think measuring leg length difference like a PT does (pulling your legs together while you lay flat on the table) may be different than what the surgeon does (x-rays and fancy resurfacing templates and tools). I'm speculating here so you might want to ask Gross about what exactly that measurement means and how it's measured.
I would think measuring off some solid points on the pelvis and femur using x-ray and maybe some tools during surgery, would give you a pretty exact measurement within the hip area. The PT on the table is going to get a lot "slop" from the back, angle of pelvis, and maybe even parts of the leg. A lot of that "slop" goes away after sugrery and after recovery from years of limping.
Also, If I recall correctly, the surgeon is trying to get the spacing back to the happy spot of about 8 mm (2 layers of cartilage and 4 mm or more of syn fluid), so they have to think about the thickness of the cup and the cap, which are essentially touching each other, and try to recreate that "spacing" which would put the hip back to a pre OA condition. I think that is the primary goal for spacing and the actual leg length difference that might be measured by a PT is not really considered during surgery. Again, I'm speculating, might want to double-check this concept with the surgeon.
I guess, my point is that I've seen a number of discussion on this topic and a lot of the leg length issues seem to resolve themsleves after surgery and recovery to a more normal gate/posture.