Assuming a 160 lb male in his 30's who has a 'typical' physique:
Average skeletal mass (including marrow) is about 24 lbs (15%)
Average muscle mass 64 lbs (40%) and 28 lbs fat (17%).
Plus 10-12 lbs of blood, 3 lbs brain, 3-4 lbs lungs....etc.
RDA for the above average male is 50 grams of protein, but that's just for maintenance. Once he starts bodybuilding his _muscles_ will need more protein, but the rest of him won't (or will need minimal extra amounts). If our 160 lb guy bulked up to 220, he doubled his muscle mass, and I know quite a few 180 lb guys who carried less than 40% muscle (I was one, egads)
I know recommending protein based on fat-free mass is common, but what works for a guy carrying 120 lbs of muscle with lots of myofibrullar hypertrophy (thus a much higher percentage of actin/myosin) is severe overkill for a 160 lb beginner with half the muscle, or less. So a 160 lb guy just starting out would be fine at 1 gram per pound simply because he's not supporting much muscle. Yet elite lifters might need closer to 2 grams per pound of LBM simply because they're a higher percentage of muscle. Not adjusting the equation to account for non-muscle mass means beginners are getting too much or elite lifters aren't getting enough.
I dunno, but I think 50+(LBM-100)*2 gets you a more realistic number that would apply to a much broader range of individuals, without being too difficult math-wise.