The dating on the ultrasounds is based on the measurements of the baby at and then used against an average size for gestational term. My last set of scans have 4 dates on the one day. My dates had it as 15w4d - BPD made it 15w6d, HC made it 15w5d, AC made it 16w0d and EFW and Fl made it 16w1d. So there is anything up to 4 days difference based on the one scan.
I remember reading somewhere that the accuracy decreases as they grow, so I agree with ticklish use the earlier scan dates