The excessive protein intakes (300+) originate from drug using bodybuilders. A bodybuilder taking drugs is going to synthesize muscle proteins at a faster rate than a person with normal physiology. Do they need 400 grams of protein per day though? Unlikely, but I can see a mechanism by which they would need more than the rest of us.
My rationale for higher (1 g/lb of bodyweight) is that there is no harm in overshooting (there is no tolerable upper limit (UL) for protein in healthy adults) and high protein diets have other benefits besides optimizing muscle gain. Insulin sensitivity tends to be higher (which could translate into better macronutrient partitioning) and is more satiating than carbohydrate or fat if you have trouble losing fat or keeping fat off.