When you hit your brakes you are at a minimum lighting up 4 additional lightbulbs (1157's I believe) at about 23 watts each. if you have a voltmeter mounted you will notice nearly a 1vdc drop, from 14.4vdc to about 13.4vdc.
Increasing your engine idle speed manually a little will cancel this effect.
It is nothing to worry about, you are still producing enough amperage to power the bike and charge the battery.
Much better than my old bike, it would drop below 12vdc at idle with the brakes on :!: :shock: