Brain at 20 week scan

I had my anatomy scan at 19w6d. Everything measured right on track. Baby overall measured at 19w3d. When she measured the brain it was saying 18 weeks. I asked if brain and heart were normal and they said yes. It just seemed odd that the brain seemed to measure almost 2 weeks behind. Anyone else have this happen? I'm slightly worried that something is wrong and they just didn't tell me or it's too early to tell.