Your cycle length can make a huge difference to your due date.
Your cycle is made up of two lenths of time, the first is the time between the first day of your last period and ovulation and the second between ovulation and period. Most calculator assume an average period length of 28 days, which is 14 days till ovulation, then 14 days till period. The phase between ovulation and period is called the luteal phase and doesn't typically vary in an individual and usually will be 13/14 days.
Where it all gets a bit interesting is the fact that ovulation quite often doesn't occur on day 14 - and can vary in an individual depending on many factors - stress is the main one.
My cycle is an average of 31 days, but over the last six months has varied from 27 days through to 41 days. From monitoring my body really closely I believe that the month I got pregnant I didn't ovulate till day 24 of my cycle...this means that if I use an online calculator with a fixed cycle of 28 days, my EDD comes out as about the 17th of March (LMP 10th June) but if I calculate cycle length based on ovualtion on day 24, add on 13 days (my luteal phase) then if i hadn't got pregnant, this would have been a 37 day cycle for me. Using a pregnancy calculator that I can adjust cycle length to 37 gives me an EDD of 26th of March.
Whilst it doesn't seem a great deal, it is when you consider that if you go overdue you'll need to consider being induced but without an early dating scan or knowledge of your own cycle length, you could be induced a week before baby needs to be.
Phew! Hope that wasn't too complicated....feel free to challenge the details! Can't wait till my dating scan to see if my calculations are correct - I think the MW tomorrow is going to estimate me at nearly 10 weeks pg when in fact I'm only 8 but the scan should sort it all out.