Which ammeter to pick for best accuracy and why?
That procedure will tell you which meter happens to be better calibrated for that particular value of current at that moment in time.

The manufacturer's specifications (assuming they aren't lying) cover the full range of measurements, considering things like the absolute accuracy of the internal reference, converter nonlinearities, component matching and how all of this changes over time (aging).

You can't assume that any given meter is better than its specifications based on a few measurements.