[Amps] Grid Bias Supply followup

[Steve Flood]

As a followup to my question about the grid bias supply on the G.E. Ham News 813 amp (and the B&W LPA-1 is nearly the same design):  

The bias supplies for these amps use a 120:120 transformer, half-wave diode rectifier, C/RC filter (40uF/500 ohms/40uF) and a 500-ohm, 25-w rheostat load for the adjustable bias.  These supplies are said to provide zereo to -100v bias.  Yet, the settings for CW and SSB only use -19 volts and -5 volts.  Why then, is it necessary to have -100 volts available?  Seems like a much smaller supply would suffice.

Thanks, Steve KK7UV