Clarify ovulation info?

So I've read on multiple resources the the best day to bd is two to three days before through your ovulation day. Why is that? 
I feel like then the percentage cakculaton could be misleading if you are bding every other day? Like you might miss the peak day.
Ha sorry if that's confusing...just trying to keep myself busy and resist the urge to pee on a stick during my tww

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