Conflicting ultrasounds- technology?

On Wednesday I had an ultrasound at my doctor’s office. They saw a sac+ fetal pole/baby but was measuring a week behind (5 weeks 5 days when I should be 6 weeks 5 days). There was no heart beat. This is concerning considering I have had 5 prior miscarriages (age related) but they said it looked fine, was not diagnostic of a miscarriage, and that my dates could be off. I have a follow up with them next week.

The very next day on Thursday I had an ultrasound at a free clinic. They saw an empty sac and called it a blighted ovum.

I’m confused but have never had this experience before. Any input?