I'm not discussing about the proof, but about why.
After my research tonight, I can explain my own question. Let's see this.
base \(1\) or unary => \(I.III... = II.\)
base \(2\) or binary => \(0.111... = 01.000...\)
base \(3\) or ternary => \(0.222... = 01.000...\)
base \(4\) or quaternary => \(0.333... = 01.000...\)
base \(5\) or quinary => \(0.444... = 01.000...\)
base \(10\) or decimal => \(0.999... = 01.000...\)
base \(16\) or hexadecimal => \(0.FFF... = 01.000...\)
and so on.
What's that mean? Now, let's find a job, yes, really job. If you can warranted that you always do the best on your job level, your boss will automatically think that you must get (absolutely get) new level on your job.
so, it's enough to explain why that happen.
"if we have maximum value on a level, it same as we have minimum value on next level." by: YIS (myself)
\(0.999...\) like the border or a limit on a level to level up, \(1.000...\) like the border or a limit on next level to level down.
For your attention in this discussion, I'm very appreciate it. Thank you very much.
another explanation, can be posted freely inside comment.