A 18-month-old girl is brought to the pediatric outpatient department for routine growth monitoring. Her current weight is 10.5 kg (plotted at the 50th percentile for her age), and her length is 76 cm (plotted at the 25th percentile for her age). Her head circumference is 46 cm (plotted at the 10th percentile). At 12 months, she was at the 50th percentile for weight, length, and head circumference. The child was born at term with a birth weight of 3.2 kg and length of 50 cm. Developmental milestones are age-appropriate. What is the most likely explanation for the pattern of growth observed?