??? Confused

I just noticed something which I am baffled that I did not notice before. Glow is saying my period is supposed to come on 10-25-14. But my periods usually come on every 19-21 days. So by my calculations it should be here somewhere between 10-19 thru 10-21 not 10-25. My last period went off 9-30-14. See what I mean...someone help.