Days Past Ovulation???

Hadley • 19 • Taken • Mommy to an angel baby
This may be a REALLY stupid question;
Why is pregnancy calculated using DPO?
If I have sex way before I'm ovulating, but I'm supposed to wait 10dpo to test, why can't I test sooner? 
If I have sex when I'm NOT ovulating, what does that have to do with days past ovulation? I really can't seem to understand lol. 

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