Implantation

Taylor

This is my 2nd pregnancy so I feel like I should know the answer to this. Maybe someone can explain. But if implantation has to take place before you get your positive test, and that happened last week for me (week 3), then why does the app and week 4 video say implantation starts this week? Am I a week further than it suggests? Or is it just because implantation happen between weeks 3-4? Thanks for your explanations!