Really confused about due date calculators!

[ScrummyYummyMummy]

I don't mean to sound stupid, but I'm really confused about what my edd is. I've tried loads of the online calculators and my midwife had a guess, but it all comes out different.
I know the date of conception was 13th October and last period was 22nd Sept.

Feel really stupid as this is my second. I'm supposed to have the basics sussed now aren't I?

Prior to your dating scan, they'll basically count 40 weeks from your last period and whatever that date comes out as is your EDD. They adjusted mine at the dating scan to 5 days later than the original one which worked out right as I ovulated around day 18/19, not 14 as most calculators assume (I have irregular cycles). The first 2 weeks that they coulnt you haven't actually conceived (unless you ovulated on the first day of your period).
HTH