Z_positions are the points of the joints where the bed-plane “crosses” the respective Z spindles. It is not the point of the arm where it connects to the bed.
See my illustration here (triple Z motors and 0,0 at right rear corner):
Some of those values can be negative as those are mostly outside the printable area.
The points under the z_tilt section are the coordinates of the nozzle closest to the Z_positions.
Those points normally are within the printable area.