Glow Ovulation Prediction

Ne

I put in my OPK results (surge or no surge) into the Glow log and it updated the calendar to indicate ovulation on April 15.

The surge day I put in was April 12, and 11 and 13 were both negative. Doesn't 15 seem very late for ovulation after the surge?

Edit: I also logged a surge on April 9, but got negatives before and after that until the 10th...but wouldn't that have brought my ovulation date up if anything, not pushed it back?