Why is it so hard to answer this question?
How much lean muscle can the average guy put on and how much can a gifted guy put on?
We're aware that Ronnie has better muscle genes than the average gym rat, but how much?
How much faster in the 100m dash is Usain Bolt than the average athletic guy in his prime? 25-30% maybe.
If this is also the case in muscle building than the difference is a few pounds and the chart is correct.
There can't be any kind of accurate formula for figuring out how big any one guy can get, because there are too many variables to consider. It's much more than just genetics. It's also diet, exercise, rest, mindset, etc. You can have the best genes for bodybuilding (whatever those genes are, as they haven't been scientifically identified), and if your diet sucks, your exercise is stupid, your rest time is inadequate, and mindset nonexistent, then you won't get big in bodybuilding anyway.