Back in the Reagan years--early on they traveled to England to meet with their leaders and Queen Elizabeth II asked that they come by as well whilst there. Prior to the meeting, Mrs. Reagan was apprised of British customs in these situations. A lady showed her how to "curtsey" to the Queen when she met her. Nancy Reagan said to the woman--"I am the First Lady of the United States and the wife of the President--I don't curtsey to ANYONE." Decorum is everything.
She knew how to handle a "foreign" tradition and customary greeting.
I guess Obama made a slip-up based on some "prior experience".