Required accuracy / resolution of truck scales?

All I got to add is that lathe is super cool!

There is such a thing as "Full Scale Error" and some gauges are rated that way. So the gauge may be rated "5% FSE" which means, obviously, that it won't deviate from true more than 5% anywhere on the gauge. Those gauges cost a bit more than a std. gauge. Presumably scales can be had the same way. Whether or not these particular scales are rated that way is unknown.
I'd expect that balance beam scales are long gone. These scales are likely electronic, so the error is more in how the calcs are done than the strain gauge itself.