It's not to do with hypos Alan!
'They' reckon 'we' take far longer to make up the blood loss. This is actually untrue but those are 'their' rules.
The Wolfson Unit at Bham University who are responsible for the QA in all the labs that do our A1cs, are fuming about it, but they can't get them to change their minds despite various T1s and T2s on insulin having been initially allowed to give blood specifically for that purpose, and not suffering any harm. I can confirm that I was fine, and I loved it - I hated having to stop doing it when I was diagnosed. Also normally I'd have been considered too old to start - but as it was just for this and because I'd been there done that before, they said OK.
Furthermore you do have to be careful not to have your A1c done again within at least the next 3 months after donating, because you will have more NEW blood cells than normal following it, so you'll have got rid of some of the glucose stuck to the haemoglobin in the pint you gave .......