I wouldn't worry. Both my kids had head circumferences that were miles away from their height and weight percentiles. DS was 50% for weight/height and not on the chart for head and DD was 90% for weight/height and under 50% for head. They, like me, are just small headed people.

Also they are really wriggly and with different people doing the measuring in slightly different spots, even a cm difference makes quite a big change on the graphs.