I did manage 5.2% on my last one about a month back, down 0.1% on my previous one!
My (non scientific) thoughts are that there is some figure or another that your body will allow your BGs to go down to, below this and it starts to fight back, I have, in the past, strictly restricted both calories (1000 a day) and carbs (30 gm a day) and lost a great deal of weight, but even in the middle of this strict regime my HbA1c never dropped below 4.7%.
I think (non scientfic) that your body simply starts using stored fat and converts it into sugar and keeps levels somewhere near where it wants to be.
As to the figure at which we start using fat, I guess we all vary. One thing I do know is that if I eat to much carbs my body sure knows how to make that in sugar PDQ!
It is a comforting thought that your (and mine
) levels are close to non diabetic and might even stave off those dreaded complications for ever. But then it might not.
H