Changes were made in the 2011 NEC. Found the following explanation (extracted from a longer article about the 2011 changes).
"The ampacity of some conductors in Table 310.15(B)(16) (formerly 310.16) didn’t match those found in the Canadian Electrical Code and were therefore changed. While no technical evidence was submitted showing insulation failure of the conductors, this proposal was passed. The end result was a change to the ampacities of:
Copper conductors: 14, 12, 3, and 1 AWG, and 600kcmil, 1,500kcmil and 2,000kcmil.
Aluminum conductors: 12, 8, and 6 AWG, and 300kcmil, 700kcmil, and 800kcmil."
