They may be going off her last well visit appointment, possibly before she started Kindergarten. They also often measure them at school, my kids always came home with some "me' book from the beginning of each school year, "My heights is______, My weight is______, My eyes are________, My favorite food is____.
The weight is actually about right for a 5-6 year old female (I think it said 45?)
Another consideration: She's 1/4 Filipino. My kids are 1/2 Filipino and in my girl that translated to always being in the 1st-3rd percentile for height/weight. Teeny. She was wearing 3T when she started Kindergarten. When you put Asians on a U.S. Growth Chart, they generally fall on the small end. In the Philippines, my daughter would be considered quite average in height, while she was growing, and now. At 13, a she's finally made it to 5 feet. They think she's about capped out in height. She's taller than her Filipino grandmother who is 4'10", and weighed 87 pounds the day she got married....at age 26!