Let's step back from the alphabet classification system and ask a different question: what is it that they are classifying with these letters?
We're assuming that "Class M", versus "K Type", etc., is a broad classification of all of a given planet's characteristics. Why assume that? I always looked upon Spock's reports on planets being "Class M" or whatever as an indicator on what kind of gear a landing party would have to wear (assuming surface conditions allowed for a landing party). So "Class M" would mean normal Starfleet fatigues were in order. By the "K Type" report Norman gave in "I, Mudd", it sounded like Muddworld and Elba II were similar in that they would require space suits or pressure domes. (But Marta was able to live long enough exposed outside to be hauled out and blown up.)
So maybe the Alphabet Soup was a handy indicator for how starship captains should outfit their landing parties or colonization expeditions. Consider it a "life support" indicator. Probably a standard developed by ancient Vulcan space travellers to determine whether or not it was worth landing on a given planet.