While Mary King isn’t the only person that I am researching with a birthdate that is not defined, she is the first one I found that lost 10 years between death and burial.
Perhaps the reason was an error, perhaps it was because many people believed a wife should be young than her husband. We know with Mary King and Joseph McKee this was not the case, as their marriage certificate lists Mary being of “full age” (over 21) and Joseph as being a minor.