How is Weeks of Pregnancy Accurate

My boo creampied me twice on the 13th of this month ( was ovulating and didn't know ) and i took a plan b the next day.

i found out that if you took one WHILE ovulating that the pill wouldn't work because it's meant to delay ovulation to avoid pregnancy ( correct me if im wrong )

so, I started toying around with how far i would be if im actually pregnant. i will find out in 2 or 3 days.

the calculators online say I'm 3 weeks and 4 days but HOW is that even possible when it's been just a week and 5 days since it happened? 🤔🤔🤨