Why do some baby websites get your due date and week all wrong?!

So I go to said baby website and I plug in my info and it comes up with some insane due date and says I'm a week ahead of where I am? What is UP with that???? My favorite website is www.babycenter.com. It has all my information right and I love the information they send. Sorry if you are a fan of What to Expect but I don't like that book and I don't like that website. Everyone is different in what they need and I guess I need more of a lighthearted approach to what I'm going through. My sister has said that Girlfriends guide to pregnancy is great too!