[ RadSafe ] Difference between DOT and NRC Specific Activity Values for U-Nat

[Peter Collopy]

Looking to find out why the NRC has a listed value of 6.77E-07 Ci/g and the DOT has a value of 7.1E-07 Ci/g.
Al;so found an entry on an ORISE pdf for U characteristics that lists the U-nat SA as 6.91E-07. 
I know they are not much of a difference but hope to get soem feedback on why they calculated different values.
Pete C