I dont, know for sure but it sounds like the 1820 one includes the garage sf which is incorrect. Garages dont count towards Gross Living Area.
Both of the diagrams do not include the garage area, but both diagrams do include the converted garage bay as living area. I believe that is the right way to do it, but correct me if I'm wrong.
There are a few measurements that differ on the two diagrams by a foot or two, but I assume that is fairly common. The major error is that the recent appraiser misreported the length of the house by 20 ft or so. I triple-measured my house and I'm sure that he got it wrong.