My guess is that the uncertainty has little to do with gravity, although there is gravitational variation in both the Moon and Earth depending on what part you

My guess is that the uncertainty has little to do with gravity, although there is gravitational variation in both the Moon and Earth depending on what part you are close to but I don't think that comes into play here. The error is likely more to do with the observational data. Try pinpointing the exact distance and trajectory of a 1m rock using ground based telescopes. Hard enough but remember there is also atmospheric distortion and you only get limited time on the scope.