One suspects that Herself has a serious anger management problem, and the appearance of the fixed smile or grin is a psychological trick taught to Herself as a way to mitigate the outburst of that sometimes explosive rage. This is almost as effective a signal to others as the characteristic “tell” of a mediocre poker player, which the other players at the table may use to “read” what cards he holds.
And Herself has exploded with rage rather spectacularly on numerous occasions in the past.
“What difference, at this point, does it make?”
She did explode—it manifested itself in the “na na na na I have inside information” blunder when she leaked the four minute nuclear launch timetable.
Her temper is well-known.