After many years of using ImageJ I only now realized that the counting for the z-planes is inconsistent. In the Hyperstack the first z-plane is 1 and in the ImageJ Menu bar the mouse-over gives z=0. Probably nice to have it consistent?!
Coordinates, including z coordinates, are zero-based. Channel, slice, and frame numbers are one-based. It’s too late to change, but it probably would have been better if channel, slice, and frame numbers where zero-based.
OK! Thanks for the answer!