Early scan dates????

Kiara • Pregnant with baby #1, Due May 2016
Hi ladies, had my first scan today making sure everything was going ok, didnt see heartbeat but been told to go back in 7-10 days to check again.. my dates from my LMP i would be 5 weeks 4 days today, but ultrasound measuring 6 weeks 5 days??? A little confused. Thanks in advance