Measuring behind when I was measuring on schedule.

I had an ultrasound 2 weeks ago due to abdominal pain. I was 19w3d based on LMP and was measuring exactly on track with my due date. Today I went in for my anatomy scan and was measuring 20w6d when I should be 21w4d. I find it odd that baby didn’t seem to grow much from the last ultrasound. My first ultrasound I ever had for this pregnancy, I did measure 4 days behind, which was done at the same place as my anatomy scan. Could it be that there is a difference in machines? I’m just worried my baby isn’t growing how he should be now. Anyone else have a similar experience?