Clear blue opk not very accurate

Okay ladies I've been using both pregmate and clear blue digital opks for the last 2 months. I've wanted to make sure that when I O'd that I actually did so I wanted to use both. But this month on Thursday my cheapie wasn't a positive but my clearblue digital had a smiley face. I took them both again Friday and they both came out negative but my cheapie was still getting darker, so I decided to test again on Saturday. It was still a negative on the clearblue but my cheapie was still continuing to darken. I then took one today and I got a positive on the cheapie and the clearblue! If I wouldn't have continued testing I would've thought I ovulated on Thursday/Friday & I would've missed my chance! Don't just rely on the clear blue guys!