Regarding the precision and accuracy of Atomic mass

[Sebastiaan]

The website of CIAAW list the 2012 atomic mass in Daltons (Atomic Mass Units) and mention the atomic mass of stable isotopes followed by an additional number between braces. For example Helium-4: 4.002 603 2541(4)
My first question is what does the number between braces mean? I assume it is some uncertainty, but how to interpreter it exactly? Secondly, why aren't these number used at many other sources which appear to use the dated 4.002 602 which originates from 1983.

 

[Bystander]

Secondly, why aren't these number used at many other sources which appear to use the dated 4.002 602 which originates from 1983.

Welcome to the wonderful world of "standards/conventions." As to "Why," long as you cite your sources in excruciating detail, you cannot be faulted.

 

[Sebastiaan]

Alright, I think I understand the brackets after another number means a bit better, it means the standard uncertainty expressed in terms of the least significant digit, so the (4) in 4.002 603 2541(4) means a standard uncertainty of 0.0000000004