Baby's head circumference was 93rd percentile at 4 months, 88th at 6 months & 99th at 9 months. it was measured by different people every time. doctor says it's normal. can this be too much of increase between 6 & 9 months?

You have nothing to concern, trust your child's pediatrician.

Not really. Head circumference (HC) increases 1/4 inch per month after 4 months of age. Therefore the head circumference in 9-m-o infant should be around 0.75 inches above that of 6 month's. It is estimated your infant head had grown 0.2 inches extra in 3 months which is considered insignificant. I assume your infant's HC was well above 50th percentile at birth.