Confused between measuring behind and percentiles

[Confused289]

Hi

I had my 20 week scan, I was meant to measure 19w1day but was measuring 18w1day. The ultrasound tec said sometimes that can happen but my percentiles are particularly small (below 10).

I see some people post about being 2 weeks behind and in maybe 17th percentiles. How can I be only one week behind but measuring between 4-9th percentile on measurements?

Are they using personalised growth charts? I don't know where you are based but in the UK the centiles will be different from mother to mother as they take personalised data