If I may be so bold as to offer my own interpretations...
Confidence in brush strokes is an expression of stroke economy. You use less brush strokes to do something, and as a result it's both clearer what you intend to actually draw, and looks cleaner and more finished as well.
Quoting from the guy who did the very excellent PSG7 tut
Originally Posted by Niklas Jansson
Each stroke should be contributing to the piece. Some hope to get away with random artsy strokes. Random strokes makes random details. Random details distracts the viewer from the actual details of the painting or drawing.
That way, if you can express an unimportant area in fewer strokes, you will have more freedom to detail in the focus of the piece.
Happy accidents are exactly what they sound like, fortunate little details you didn't mean to produce, but happened anyways. Hard brushes tend to produce small bits that, due to either context or imagination, create a small detail that enhance the verisimilitude of the piece. Soft brushes just mix things into an amorphous blob of indeterminate form.