With a FBG of 90 and an HbA1c of 10, what is likely happening is that the person has sufficient insulin functionality to bring glucose into control during the night when no food has been consumed for the fasting period, BUT that when food is ingested blood sugar is rising VERY high, insulin production is badly failing to keep up, the liver may actually TURN ON glucose production making the problem worse, and EVENTUALLY insulin catches up bringing blood glucose down. If this happens every time the person eats, their long term average blood glucose (HbA1c) will be very high, but fasting glucose can be low.
On the other hand, it may indicate that the blood sugar levels were totally out of control in the previous three month period and now there is better control.