Your GP could be refiring to "Hb A1c", glycaemic control of haemoglobin (red cell).
Basically, the Hb A1c reading of 8 means each red cell has a high concentration of glucose attached, and different to the test you do each day.
A reading between 1% to 6% is ok and 8% is moving into the high side. 1% range = very athletic person.
To get an accurate reading you need to fast before blood pathology test in the morning.
Diet and walking an hour each day is the main way to keep on top of it that you say you do, however, the GP may prescribe a pill called Diamicron etc later if the level become a concern.
Why is this happening, could be genetic.
My parents are diabetic, probably diet caused from what I know. The cost is not in the moment but many years later.
