understanding my scan

Nicole
Hi everyone. I've just been for my 4d scan and looking at measurements I'm a bit confused. I'm 26week + 4. It's says 
GA (EDD) - 26w4d
GA (AUA) - 25w5d
EDD - 21st September 
EDD (AUA) - 27th September 
Does anyone understand this at all? X x x