Calculating my due date.

Hi ladies!! Just a general question how tf do these apps calculate your due date? Got a positive test, I know for sure the date of conception was July 25th. My last period started July 15th lasting about 4 days which is normal for me. To me that’s only four weeks and 3:4 days. Somehow everything is telling me I’m six almost seven weeks pregnant already. This is my first time and I’m just wondering if I’m missing something. Like are gestational weeks shorter? I’m so confused 🤗😬