there are different cut offs for diagnosis.
Hba1c < 5.0 is totally normal (some experts consider optimal is 4.6 - 4.8 range), and it can be lower in children.
Hba1c above 5.0 is often seen to progress, ie. get higher, even though this is not yet considered diagnosable. This means that if your hba1c is between 5.1 and 5.6, you should also treat it as an early warning to look at your lifestyle, weight, diet (lower carb is best when it comes to diabetes prevention), exercise, etc.
Hba1c of 5.7 to about 6.4 is typically regarded as prediabetic. Consider this as early diabetes and action is required.
hba1c of 6.5 or greater will get you diagnosed as diabetic. Serious action is required to normalize sugar levels.
From this you can see that diabetes and blood sugar levels are a continuum.
Many people don't feel any symptoms of high blood sugar until they have had high blood sugar for quite a long time, and then they go to the doctor because of bothersome complications. If that is the stage they are diagnosed at their hba1c can be very high. 10 - 12 or even higher.
It is much better to have regular monitoring, annual would be recommended, so you can observe your hba1c over time and catch any development of diabetes early. IF you notice that it is still "normal" but trending upwards that would be the sign to do something.
My aic was 13 when diagnosed