Is glow genuinely accurate??

Okay so I set up reminds on the days that glow told me I would be ovulating. They even told me that the most important day for me to BD was the 18th. I put into my phone that I had a 33% chance according to Glow. Well, my green days switched on me and now the 18th is now at 3%. The 18th was my last day of O week but now it was the 17th. Am I not understanding something?