Ff and ov date? Confused......

[Louise2013]

I had a pos opk on sat am and pm, and a neg on sun am and ever since, so why does FF tell me I oved on Sat? i'm not temping, but I thought a pos opk was pre-ov and assumed I ov Sunday? Any ideas? Xxx

 

[scn]

You can ov anything from 12-36 hours after the first positive opk