Clearblue OPK Digital

I am using Clearblue digital opk and accidentally started testing a couple of days early according to pamphlet in the box and my last cycle. When I tested I got a low fert reading, which I expected. I decided to skip a day to make sure I had enough sticks, but when I tested today I got a high fert reading (blinking smiley face). I was a little surprised by that, thinking it would be another low fert day. Anyone have experience with Clearblue OKP? Used them before and they worked great (got us our first BFP and baby!) but had no weird things happen. Just curious....