Hats are made with something called negative ease. Negative ease is a sizing term that means the finished measurement is smaller than the actual measurement. In this case the finished hat is smaller than the head size. You can see where I took this into consideration in the 3rd row of the chart. I like to use less negative ease when making hats for babies and children than for adults. The reason for this is a baby’s tendency to grow at an alarming rate. You spent quality time making that hat, so you’ll probably want to make sure it fits for more than a day!
If your baby has a current head size of 15” and you make a hat that fits with no ease (exact measurement) then you’re ensuring that the hat can be worn until baby’s head reaches 16.5, or perhaps even 17, inches circumference.