Problem in Z coordinators in Holistic detection

Question

When extracting coordinates X, Y and Z I get positive and negative numbers in the Z, any explication?

For example:
[X1=0.409132    Y1=0.764077 Z1=-0.008088    X2=0.468663 Y2=0.747041 Z2=0.010161]
Why Z1 ==>Negative and Z2==> Positive