Due dates confuse the heez out of me!

Soooo I pretty much know around the time that I conceived. We had quite a bit of snow in NJ and hubby and I were pretty much glued to each for about a week. I got a positive pregnancy test on Friday and it seemed like I would have only been 2 weeks. But when I entered it into Glow, it tells me I'm 4 weeks and due around 10/13. I have some insurance issues to work out with my job so I can't even get to a doctor until that's resolved. I'm just anxious to know and I love reading about the development stages but wondering if the stages appropriately apply to me. Anyone else mystified by how pregnancy is calculated? ?