Why do my days keep changing every time i get a ultrasound?

When I took my sonogram they said I was 6w3d I went for a second one I believe at 12 weeks but it pushed it back two days from what they had told me the first time ...I had one yesterday thinking I was 19w6d and they told me I was 19w3d why does it keep changing which one is right...lmp was 9/7/10