wwhitney: Yes, I forgot that the basic NEC definition of ampacity involves maximum CONTINUOUS current not just maximum current (more specifically "Ampacity: The current, in amperes, that a conductor can carry continuously under the conditions of use without exceeding its temperature rating."), so thank you for being patient in reminding me of that.
I read an explanation of some of the reasoning behind Article 210.10 (A)(1) (...the minimum branch-circuit conductor size...shall have an allowable ampacity not less than ...125 percent of the continuous load...), which readers of this thread might be interested as well, although I am not certain that it applies with a circuit load connected via a receptacle.
Briefly, "The manufacturer [of a connected electrical device] ... [may] count on a relatively cool conductor to function as a heat sink for heat generated within the device under these continuous operating conditions.".
This is a subtlety that I, for one, rarely think about. However, I am all for better understanding of reasons behind important safety rules.