Why was the A319 given the "19" designator? I realize that on the surface it makes sense because it is shorter than the 320, thus giving it a "shorter" number.
But on a purely mathematical level, isn't the A319 part of the A310 block of numbers? So in that narrow, technical sense, it can be argued (albeit in vain) that the A319 is actually a member of the A310 "family"?
If the A320 "family" is A32X, (20-29), then shouldn't the A310 "family" run the gamut of A31X (10-19)?
Why not make the shortened version the A321 and the lengthened version the A322 just for numerical continuity?