Ok so I got an ultrasound and was told I was 5weeks 6days with a due date of 8-18-18 (this was on December 22)

I got another ultrasound when I was supposed to be 8 weeks but I measured 8 weeks 1 day, so they moved my due date to the 14th (this happened on January 6th). This doesn’t make any sense to me. I was only 1 day off from what they told me originally but they moved my due date 4 days?

This was 2 different doctors who gave me my due date so maybe one did the math wrong? Does anyone know how this could have happened? Thanks