confusion with dates!

Abigail
Can someone help me please...
So at the doctors and on this app they get our due date from the date of last period! 
Dependent on how late we conceived does that depend on our due date? I'm seeing a lot of girl with positive tests 10dpo it took me into my missed period to get a positive! I'm 4w5 now and my test is still not that dark? 
Is that possibly because I think I ovulated later?? 

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