The 100lbs scale would be much more accurate. Many scales are not accurate at their extremes especially as they get older. I would not be surprised if your float pulls a good bit over 50 lbs and the scale simply doesnt read it properly. If you have some gym weights you could get a rough estimate of the buoyancy and check your scale.
When I asked about the bladder dimensions that was an error on my part. I was trying to refer to the portion of the float that actually inflates. The dimensions you gave me seemed like overall dimensions and there appears to be at least an inch in all directions that does not inflate.