Help! Different results for clear blue digital OPK?

I have been using the clear blue digital opk for a week now. I got a low fertility (empty circle) on the 21/2 and then flashing circles everyday since. I bought another packet today because I am away for Work and only bought two tests with me. I decided to use the new digital tester and the one I had been using to test the same urine and see what results show up.

The old tester (the one I have been using since the 21/2- the one with the black mark on it ) shows high fertility- not peak (flashing face) and the new tester shows low fertility.

How is the possible? I have zero faith in these tests now!