the only answer is that “it is defined this way in 3GPP TS45.005, chapter 4.1.1”
there is no “other way”.
class 4 means 33dBM. That is the sole and only definition of class 4.
If they had called it “brown sugar on fingers” instead, then that would be how we call MS whose txpwr=33dBm. And “class 4” would not exist.
regards
pix