I think it's a good question. I have often asked myself that. The only conclusion I could come to based on some research is that Va is obviously based on the structural integrity of the aircraft for abrupt control movements also related to severe turbulence. Rough air penetration speeds are related to passenger comfort but more importantly aircraft stability based on the abilities of controlling the aircraft in moderate or greater turbulence. It was probably determined during initial flight tests. I noticed that the CR7 is a much more stable and a comfortable ride when you slow it to .75/280kias whichever is lower when you encounter moderate or greater turbulence. That number is a constant where as Va changes based on weight and altitude. That could be total B.S. but it's a start.