How many weeks pregnant?

Why does the <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow app</a> say that I am 3 weeks and 6 days pregnant, but every other site I calculate it on says I am 4 weeks 5 days? This includes the P Tracker app that I have been using for two years and has longer records of my cycles.