We currently assume an uncertainty interval of +/-1 or the least significant digit of a given quantity:

10.2 => 10.2 +/- 0.1 => interval [ 10.1, 10.3 ] => uncertainty magnitude 0.2 13e3 => 13e3 +/- 1e3 => interval [ 12e3, 14e3 ] => uncertainty magnitude 2e3

This means the *magnitude* of the uncertainty interval is *twice* the magnitude of the least significant digit. It seems more intuitive to assume the size of the interval to be one order of magnitude:

10.2 => 10.2 +/- 0.05 => interval [ 10.15, 10.25 ] => uncertainty magnitude 0.1 13e3 => 13e3 +/- 0.5e3 => interval [ 12.5e3, 13.5e3 ] => uncertainty magnitude 1e3

This is also supported by the Wikipedia article on the quantification of accuracy and precision:

https://en.wikipedia.org/wiki/Accuracy_and_precision#Quantification

Perhaps the current implementation came about due to confusion about the magnitude of the uncertainty, and the +/- notation.