The second bottle has little particles of Zinc in it that settle to the bottom. You really have to shake it to get those particles back into the solution.
The problem is that if you dont shake it enough, eventually the concentration of zinc particles increases in the bottle and the test starts to give really high results.
I always replace the test kit when it gets about 1/2 empty and the old test kit always reads higher, sometimes by twice as much, as the new test kit.
So be diligent with the shaking. I also wack the bottle a few times on something hard to help mix up the particles.