due dates given from doctors are an estimation of when you got pregnant.
they go from your last period and work out when you ovulated, and when you should be due.
but i would think they go by the average 28day cycle, and not everyone has a 28day cycle and ovulates the same day as every other woman, so its just an estimate really.
some women can be really close from the due date they recieve, but its still all estimates.