Hemoglobin is glycosylated at any concentration, even normal blood sugar levels. This is why there is a "normal" hemoglobin A1c range.
The problem comes when there is an elevated blood glucose. The problem is with the elevated blood glucose, not that there is an elevated Hgb A1c. The A1c is only a marker and a way for physicians to measure the average blood glucose over the past 120 days.