A couple things here (if you are talking about looking at your average BG on your meter). On your meter, a low BG will average out a high BG. That is reflected on the number it shows on your meter.
A 40, 40, and 200 average out to 93 on your meter. But you still had the 200, which elevated the amount of glycation you had, despite the 93 average.
Also, BG meters are not a random sample of values! You don’t test at random times everyday. You test when you eat, when you feel bad, when you are high, when you are low, when you are taking insulin. So the BG meter average is only an average of the times you have tested. I know that seems like a dumb simple comment to make, but it really is relevant. I know that I test much more when my BG is bad compared to when it is good. I think that is a very common thing to do…
The A1C, it is not a measurement of average. So lows don’t reduce it. A1C is a measurement of the amount of glucose that has bonded to the hemoglobin. So “average blood sugar” when used to describe A1C is somewhat over-simplified
You can get somewhat of a ballpark idea of your RBC lifespan by getting your reticulocyte count and doing the calculation. It is still a ballpark, but it can give you a little better guess of your RBC lifespan, and how it might compare to normal lifespan.
But with a 5.1, you can be happy. Even if you had a very short RBC lifespan, a 5.1 is great no matter what. So congratulations.