you could have had a delayed ovulation which would push that date back.  Also consider as a contributing factor that it may have implanted in the later  half of the window.

did he express any concern? was he able to see the sac and fetal pole?

Right now I wouldn't worry about it as long as you aren't experiencing any pain or bright red blood...

It does sound like from what you say the doc said that you probably had longer follicular phase (first half of the woman's reproductive cycle) while the medical "norm" is consdiered to be two weeks (ovulation occuring Cycle day 14)