Why are the above values of initial-starting current and torque approximate?

Because the standstill/reactance of a motor is constant at all voltages -that the short-circuit current varies in direct proportion to the applied voltage. Owing to magnetic saturation, particularly of the slot lips, the standstill reactance tends to be less on full volts than on reduced volts so the current and torque values tend to be rather less than those obtained by the formulae given.