I don't understand...please help!

Dr
I don't understand how you can have implantation bleeding as early as six days after conception but your not considered truly pregnant until three weeks (which is well over the six to twelve day implantation bleeding window) because three weeks is when the baby implants to your uterine wall.
I'm so confused, they don't implant twice so why is there only a six to twelve day window. Three weeks is well over 12 days!