I am on the opposite end of this. My 20 month old has been way above the 95% on height and weight. She is now 34 1/2" tall and 32 lbs. She wears 3T's in clothing, size 7-8 shoe! She is the size of the ave 3 year old.
At 9 months she grew out of her carseat and we had to move to the booster seat (in the car backwards of course). She met the height and weight requirements to turn her face forward but she just wasnt a year yet.
At 12 months she was wearing 2T - 3T's!
I get a lot of grief with people telling me that she is over weight or comments like "Boy, i bet she never misses a meal". In all honesty she doenst eat breakfast or morning snack. She eats a big lunch, afternoon snack and a big dinner. She drinks 4 oz of juice and 30 oz of milk a day. She eats a balanced meal and no sweets at all not even icing on her birthday cake.
Every child is different! Each child grows at their own rate...the scale they use is for the ave child. Some children are not "Average" some are bigger and some are smaller. As long as you are feeding her the daily requirements i wouldnt worry about it.
My daughter just had a spike in weight but she also just went through a growth spirt and her Ped wants me to reduce the amount of food she gets but i havent changed a thing. i am sure you feel like you always have to give a menu to people about what you are feeding her because they dont feel you feed her enough as with mine they felt we feed her too much.
if this makes you feel better...i watch a 16 month old little girl and she weighs 20 lbs not but three months ago her feet were the size of my daughter when she was born. She had no bottom so much so she had a concave area she was that skinny. She was just like your daughter but she now has a tush :-)
Dont worry about it she will most likely always be thin...it is just who she is.