Underweight Chubby Baby?

My dear daughter had her 12 month well-baby check up today. She's growing nicely, in terms of head circumference and length. However, she has only gained a pound since her 9 month check-up. The doctor said not to be too concerned with her slow weight gain because she looks healthy. In fact, our daughter has the cute rolls of fat on her forearms and thighs. She seems to be of average weight for her age but, given her progressive weight gain through the first 9 months of her life, the doctor had expected her to weigh more than 20 lbs.

I found the slight contradiction rather strange. She's a chubby baby - double chin and rolls - but she's relatively underweight compared with other babies who have grown at the same rate as her from birth to 9 months? I'm not too worried about it but I wonder how accurate such growth charts can be. After all, adults have a weight range, usually a generous 20-25 lbs. difference between underweight and overweight. How can we subject babies to a stricter guideline? Shouldn't we consider the baby's size (naturally thin or "big-boned") and his or her parent's size (small, short parents usually have similarly shaped children). I'm 5' 3'', 125 lbs. My husband is 5' 6'', 170 lbs. We're relatively small and short people. Isn't it just natural that our daughter would fall on the thin (or light) side as well? Just a thought.