Before worrying too much about it, I would use another scale. If the area around the scale isn't perfectly flat, you will get the results you describe. My guess this is the case and the reason for such a large variation each time you weighed. Leaf springs have a good bit of resistance in sliding between each leaf which can cause them to not flex and hence carry more or less weight at times. Depending on how you moved across the uneven surface, would depend on if you had enough force to cause the springs to slip and equalize. With as little as 1/2" variation in the ground could cause the results you see.
In regards to the one leg being off the ground when you have the same dimension, I would suspect a combination of the ground being uneven and possibly even the jack being mounted a slightly different heights. With as little as being out of flatness by 1/8" would be enough to have a rear jack where it was off the ground. It would take a laser level to determine the flatness as a bubble level isn't precise enough.
Like Grit dog says, sounds like you are worried about a problem that doesn't really exist. The only numbers I would really trust are the axle weights.
On a side note, I drove my family crazy spending 30 minutes on one of those Oregon scales on a cross country trip. When I found one, I measured my FW and truck weights getting every axle and individual tire weight. They fussed and I simply told them it was one of the things I was looking forward to the whole trip and after driving 3,000+ miles, I was going to have my fun.
