Clear blue ovulation kit

So this month I decided to use clear blue to track my ovulation. Cd11 and 12 were both empty circles, hence low fertility. This morning, on cd 13 it came out as peak, a solid smiley face! How could I jump from low to peak? Has this happened to anyone? I never got a flashing smiley face to indicate high fertility, so much for their 4 days of high fertility advertising!