Dates or off from ultrasound 🤔

Daisy

So as we all know our growing babies are measured by our LMP, in my case my LMP was the 17th June and lasted for around 12 days. When I had my 12 week Nuchal I was measured at 11w 5d which is a week behind my LMP.. could this mean later ovulation because of my long cycle? Estimated conception happened on the 6th July or possibly later. (confirmed from 2 scans). Has this happened to anybody else?