So many reasons due dates can be off.

* Date of last period incorrect or unknown
* Date of conception unknown - luteal phase varies
* Not every woman has a 28 day cycle and ovulates on day 14 which due dates are worked out on
* Ultrasounds are incorrect / are already inaccurate within a few days and are based on averages, not your baby. Especially later ultrasounds, when genes have taken over and your baby is more unique.
* Emotional issues/stress deferring labour (adrenaline counteracts oxytocin)
* Average gestation is not 40 weeks, its an estimate smack bang in the middle of what is classed as full term. 37/38 (depends who you ask)-42 weeks is full term.

Personally I think due dates should be calculated on 42 weeks. After that you ARE overdue and can discuss plans to move things along, IF the dates are correct, IF there are signs mum and bub are not healthy and IF mother feels that she is okay with it.