Due Date Confusion...

Okay.. So. My calander said that I was due to ovulate on June 7th. I got a positive opk on June 3rd, so I am pretty certain I ovulated June 4th due to yhe opk going negative. Well based on my last period they are saying I am due Feb 24th, but with the day I know I concived they are saying I am due Feb 25th? I ovulated 3 days sooner than my calander said. What the fluff? How did these stupid calculators get those dates?