it seems you are not familiar with navigation in the 3GPP. In the webpage, you see all “versions” of this document (version 4.1.0, version 5.x, version, 6, 7, etc).
choose the latest version : 10.0.0
this downloads the .zip on your computer.
In this zip, there is a word document.
go to chapter 4.1.1
With all due respect, your question is pointless, as far as i understand it. It is like asking “why 10+2 = 12”
It is just a convention, defined in this document. The MS Max Tx Power can be 33dBm, or 36dBm, or 39dBm, etc. Each max tx power corresponds to an “ID”, which is called a “class”.
I’m afraid I didn’t understand your question though.. feel free to explain 😉