Is glow wrong?

Lauren • Got my rainbow on September 26th. 🌈👶🏼 👼👼👼
Glow has me at 4 weeks and 1 day but when I count back to my last period I should be 5 weeks? I'm so confused. How do I fix it? It's driving me crazy! Lol. My last period was October 20th and my expected period was November 17th. Can any lady's please help me figure this out?! Thank you in advance!!