Low fertility straight to High fertility in less than 24 hrs

I had taken the Clearblue Advanced Digital Ovulation Test around 9:30 am on 7/20 to begin testing this cycle and got a low fertility. I woke up around 5:15 am this morning and got a high fertility. I'm wondering if since I tested less than 24hrs between the two that it's actually not accurate? My cycles vary from 29-31 days so I took it on day 8 (bc it was my longest sleep)