Why is the uncertainty of a digital instrument always the smallest scale division?
Migrating my comments to an answer:

The big issue with your line of reasoning is that the transducer in the scale does not produce decimal output. Like all digital devices, the signal is digital (i.e., binary), so the representation of the weight will also be a binary number.

Then you're in a scenario where the uncertainty is something like $\pm 0.0001_{2}$, which is $\pm 0.0625$. In this case, I would say that the error is in the first decimal place after the decimal point.

You can't give an error of $\pm 0.005$, partly for the reason I gave, but also because the error in measurement is not necessary an error in rounding. It's possible that the transducer is temperature sensitive, so within a reasonable range, it might over or under measure by $0.01$ units.