There are several "due date calculators" out there, and they all pretty much seem to give the same due date ... except in France!
See a striking example: http://www.babycenter.com/pregnancy-due-date-calculator It calculates the due date to May 26.
Then go to the French version of the SAME site: http://www.babycenter.fr/t400215/calculer-votre-date-daccouchement It calculates June 2nd.
Same goes if you go to a French doctor vs a US doctor (or Israeli, where I am).
This is science, biology, right? Why would there be a difference?