The point that is made in ISO 1925 is precisely "that for a given unbalance condition the vibration is inversely related, and force on the bearings is directly related, to the bearing stiffness". Unbalance is not shown by the external appearance but by the vibration or force it generates. A rotor may have a hole drilled in the outside. That may be there to correct the unbalance rather than being the cause of the unbalance.

How do we know the wind is blowing we feel the breeze.

How do we know if a power socket is live?

The electrician asked his new helper to pick up a cable and touch the end. He said "do you feel anything?" The helper said "no." The electrician said "whatever you do don't touch the other cable it has 440 volts on it!"

It is better to use a meter or light bulb to check if there is high voltage! The balancing machine is the meter for unbalance. Rather than putting a hand on a motor, feeling the vibration and deciding if it feels OK we need to have a way to make real measurements of the rotor condition.