Each hidden unit starts with an exact weighted sum plus a bias. The table keeps the weights, inputs, and biases visible so the pre-activations have a shown source.
highlighted = computed this step
First hidden sum
For the first hidden unit, z1 is w11 times x1 plus w12 times x2 plus b1. Here that is 2.
z1=1⋅1+1⋅2−1=2
Second hidden sum
For the second hidden unit, the negative weight on x2 changes the sign of that contribution. The exact result is z2=-1.
z2=1⋅1−1⋅2+0=−1
Summary
Both pre-activations are exact integers. The visible biases b1=-1 and b2=0 are part of the recomputed source.