Help with growth scan measurements please!


Hey guys so I'm 31 weeks + 3 days and went in for my growth scan today because my fundal height was measuring 29cm at 30w. The sonographer checked everything out and he said there looked to be nothing of concern, but I noticed that baby's head was measuring 33 weeks and his abdomen was measuring 30w+ 5. He gave me a chart too, but I can't seem to figure it out, and now I'm worried about the difference in head/abdomen. Can anyone give me an understanding of the chart. Has anyone else had this happen?