ovulating different to this app?

I've been ttc for 3 months using this app, telling me when I'm due to ovulate etc. 
I bought an ovulating kit and it says I'm actually ovulating now but this app says I'm not?! 
So confused now as to when I'm actually ovulating because my periods are always the same days apart, it's not like I have irregular periods and can't be sure when I last had one!