Measured at 11 weeks at my first 12 week ultrasound scan, they’ve booked me in for another next week, is it normal?

Went to my first 12 week scan today and they measured me at 11 weeks, so they’ve booked me in for next week for another scan for when I’m 12 weeks. Is that normal? What’s wrong with being a week early?

Also I should add my bladder wasn’t full so the quality on the screen wasn’t great so I’m not sure if this is also a reason why I’ve been booked in for another scan next week.

Anyone else had a similar problem?