I dont know the actual answer but, since the responses are few (none), I am willing to speculate. I = 34'0" is the designers number. You'd have to ask J/Boats what marks they used to define that distance. I = 34'2" is the Class rule. In order to define a dimension, the class had to determine how it would be measured. They apparently picked points that were easy to define. The extra 2" is probably just for convenience for the measurers.

Just speculation, remember.