Dates wrong from Lmp and 5+3 scan

I had a very early scan which measured me at 5+3 but my lmp was 11th December which would made me at that scan 6+3 my midwife said that seems off and we both said I should now be 7+3 days but from my scan I’m only 6+3 and I won’t know any different until my 12 week scan but I don’t understand how me and the midwife can get one date and my 5 week scan get another date a week behind 🤨 I don’t know how many weeks I am now! 🧐