My mum was telling me a story a few weeks ago about when she had my sister.
My sister was a small baby, small child and is small as an adult.
When she was growing up, the dr's would insist that my mum wasn't feeding her enough because she was so small. When she was 11 months old, they put her in hospital insisting she was under nourished, and the whole week she was in hospital, she didn't gain weight. She simply was just a small baby. She wasn't sick or anything, just small. They eventually agreed that my mum was taking good care of her and was sent home with mum.

I've never worried too much about the charts either. In fact my dr hates them. When it's time for Nina to have a check-up or immunistation, he'll weigh her, check her out etc, but he won't compare it to the charts. He's a dr, who's had 2 children, he knows if she is well or not. So do I.
Mothers instinct is best, I think.