OPK question

So how many times has your surge been later than what you thought or expected it to be? 
Last month my surge was on day  13/14 when it should have been day 11/12 as I'm a 25 day cycle, always have been. I think this cause AF to be late as I was indeed late by two days! 
If this happened last month will it again this month? I am tempting to try to get a feel for when I O.