In this article, health guru Kris Kresser says that A1C isn't an absolute test because of the different lifespans of blood cells in different people of different health. It's still always important for diabetics, overall. But, the closer to normal our blood glucose levels are, the more likely we are going to have extended blood cell lives, longer glycation period, and higher A1Cs. So in healthy people he doesn't always expect to see super low A1Cs (like in the 4s).
Kind of a catch 22, eh?
So, I guess there could be different kinds of people getting A1C measurements of 5.3. We might have those with normal glucose and long lived cells (that have collected more glycation over a longer life), and those with almost normal blood glucose numbers, but shorter lived cells. Might be splitting hairs here, but seems like either way it's accepted that it's healthier to keep them lower, as long as the risk of hypos is managed.