The complications develop slowly, & the guidelines for BG readings can be inadequate - particularly the 2 hour after meals. Try one hour after meals & you are likely to see a reading in the teens.
I was diagnosed when I reported a numbness in one leg which gave the impression that the leg wasn't supporting me - disconcerting when I was playing tennis. I followed the Dr's advice (NHS & DUK diet) until after about 8 years the numbness became crippling pain. My HbA1c was 6.7 & acceptable to the Dr. He suggested referral to hospital.
I found this forum & received the advice to cut the carbs. I did, & in 3 months was out of pain & back on the tennis court. I discovered that a carb breakfast would send my BG into the teens one hour after, but it would be down to 7-8 2 hours after, & down below 5. Fast up & fast down.
On low carb it rarely goes above 9, & I am still fit & well & active.
The numbness disappeared & has not returned. But my HbA1c is about 6.6, so the large fluctuations may give a reasonable HbA1c, but the rapid rise would appear to be the cause of the trouble.