How many days long is your cycle? Ovulation usually occurs 12-14 days *before *your next period (not after your last period, as many women believe.) So if you normally have a short cycle you might ovulate sooner than a woman with an average 28-day cycle.
Also, not all women have regular periods.
It’s possible your predictor is just wrong, but remember that if every woman ovulated reliably on the 14th day after her last period there would be no market for those predictor thingies.