Your baby's percentile/weight is based on you. Nobody's percentile chart during pregnancy will be the same.
For example, if you're underweight and say, 5ft, a baby will be higher on your percentile chart than someone who's bigger and taller.
The charts/percentile lines are estimated based on the pregnant woman.
So 3lb 12 for you, may be a 90th percentile for one person, but a 25th percentile for someone else
(An example, My first was 4lb 2 at 32 weeks and on the 20th percentile but my third was estimated 4lb 1 at 32 weeks but 70th percentile. Only difference was I weighed less)