he ISO 10605 standard specifies two different tolerances for peak current amplitude for four RC networks, as follows: +/-10% for 150 pF/330 ohm and for 330 pF/330 ohm and +30/-0% for 150 pF/2K ohm and for 330 pF/2K ohm. 

The 2K ohm type gun head has a lower inductance component than the 330 ohm type to obtain a higher first peak current value.  Note that a peaking effect (higher peak amplitude) for 150 pF/2K ohm and for 330 pF/2K ohm networks is done by the discharge circuitry but not by charge voltage adjustment.   This approach is more faithful to the intention of the standard.