"Net carbs" was invented by Dr. Atkins. His logic: fiber isn't digested, so it doesn't count - only count the carbs that actually get into your bloodstream that might spike your insulin levels. So subtract fiber carbs from total carbs, and you get "net" carbs - the digestible ones.
I think it's kind of a clunky way to do it, but that's where it came from. His recommendation of 20 net carbs/day is roughly equivalent to Mark's 50 total carbs/day if you're eating a fair amount of non-starchy veggies (like your big-ass salad, for example).