It makes perfect sense. The axle is 3" further forward. The tank isn't. :D
Where it starts to get confusing is that they both use the same frame rails, springs, trunk pans etc. So, I'm not sure why they're different at all with regard to the axle to tank distance. I was under the impression that the difference in wheelbase length was in front of the rear frame rails. But if that was true the axle to tank distance would be the same on all of them regardless of body style. I may have to take some measurements, since I have both cars.