hba1 that dun seem to correlate to routine readings

what is the standard formula.....pretty sure its a very rough estimation at best...

The Hb1AC measures an average of the BG over all time - not just when you are taking measurements.
This includes whilst your father is asleep, whilst he is watching the TV, whilst he is making dinner, whilst he is buying the newspaper, whilst he is drinking his coffee, whilst he is gardening, ... whatever he does, his body has a BG value whether he is taking a BG reading or not.
And the way the average is calculated, his BG never has to be exactly 5.0. For example, the average of 4 and 6 is 5.

millenium said:

My dad just got his hba1c as 4.7 today. Based on standard formula, his ave bgl will be 5.0. But, the readings i recorded over the past 2-3 months, i cannot understand how can it be 5.0 as i have never seen a 5.0 resding on the meter ever.

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The HbA1c can be inaccurate, and this happens mostly when the person has non-standard red blood cells that do not live for the standard 120 days. The HbA1c relies on people having red blood cells that live for 120 days, but as we know, not everyone does. Some people's RBC live for longer than the norm, and some live a lot less than the norm. If your dad's RBC have shorter lives than normal, then his HbA1c will be lower than expected. Other conditions also play a part - anaemia and thyroid issues are two common ones.

If you are unhappy with this result, I would ask the doctor to perform one of the other tests - a Fructosamine test or an OGTT.
https://labtestsonline.org.uk/tests/fructosamine

Falsely low hba1c can be caused by:

• Acute and chronic blood loss
• Haemolytic anaemia
• Splenomelgy
• Kidney failure
• Taking vitamin E, interferon alpha, Ribavirin