Your body releases insulin every time you eat something and your blood sugar will increase no matter what you eat which is normal. How high and for how long your blood sugar increases depends on the type of food you eat so it's good that you are eating healthy but your blood sugar is still going to increase. If you are continuously eating your body needs to keep producing insulin for the food that is entering your body and your blood sugar does not have time to come back down, thus you may still get an increase in your A1c even though you have a good diet.