I don't necessarily have the right or the only answer, but, I know I had a scan at 7+6 and 12+2.
At 7+6, I was 7+6, and at 12+2, they said I was 12+4.
Well, the hospital always went with my 7+6 because they said early on is the most accurate. The more you baby develop, the more you advance, the more there is variation in growth spur. So she said the earliest is the best, so they kept me at my 7+6 from that first echo!
To me LMP varies too much. If we both have a 28 days cycle but you ovulate day 10 and I ovulate day 16, and then you implant after 6 days and I take 9 days... our babies can't quite be the same days, when I'm nearly 10 days behind! But again, not a doctor, ahah. Personal opinion only!
The difference between your dates is like mine, 2 days. So usually what I do, is go with the middle (so if I'm supposed to be 14 weeks today according to first scan, and it was Tuesday according to 12 weeks scan, well, I say Wednesday is 14 weeks, haha) At this point a couple of days isn't a drastic difference in dates.
good luck with the pregnafncy