Expected Period Day wrong?

So I told my <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> that my last period started on August 7th, and that average 26 days (my last 3 have been exactly 26 days) and it is saying its cycle day 23, with 5 days until my expected period... it should only be 3 days.. why does it give me 2 more days?