Clear blue advanced digital opk- 2 tests, different results????

Okay, so this is my 8th month TTC. I have used opks in the past but this month I tried using two for comparison. I used a brand new CB digital straight out of the box and I used the one from last month as well.

The first day they both read "low" , today ( 2nd day) the new test said " high" and the test reader from my last cycle ( the old one) said " low" ???? I'm so confused, which one should I go by? Are you able to reuse the CB digital test sticks from a previous cycle???

Any help/ thoughts are greatly appreciated!!!!