Confused with head diameter help!


Ive just checked my latest dr report and see that my baby's head is at 28th percentile. Its measuring 7.72cm diameter at 31 w5 days. But at my appointment my dr didnt mention the percentile and just said it was fine because it has been measuring average each time, and showed me a graph plotting my last 3 check ups. Im so confused. I only just noticed the percentile thing... how does it differ from the average projectory graph thing?? My 26 week check up showed the babys head diametre at 6cm, which the dr again said was on the average trajectory. When i look at that report it says its 65 percentile.

Should i be concerned that the percentile has gone from 65 to 28 in 5 weeks? Or does this not matter at all because the trajectory is still bang on average? What am i supposed to focus on here?!