This boils down to the differences between the suffixes -ity and -(at)ion.
a suffix used to form abstract nouns expressing state or condition:
jollity; civility; Latinity.
In this case, plurality marks the state of being plural. Its semantic root is "plural".
-a combination of -ate1.and -ion, used to form nouns from stems in -ate 1, (separation); on this model, used independently to form nouns from stems of other origin:
a suffix, appearing in words of Latin origin, denoting action or condition, used in Latin and in English to form nouns from stems of Latin adjectives ( communion; union), verbs ( legion; opinion), and especially past participles ( allusion; creation; fusion; notion; torsion).
Pluralization is the process of being pluralized. Its semantic root stems from "pluralize".
And for pluralize itself:
verb (used with object), pluralized, pluralizing.
to express in the plural form; make plural :
to pluralize a noun.
verb (used without object), pluralized, pluralizing.
to receive or take a plural form.
And "pluralize" itself contains a suffix "-ize" that creates verbs!
a verb-forming suffix occurring originally in loanwords from Greek that have entered English through Latin or French ( baptize; barbarize; catechize); within English, -ize, is added to adjectives and nouns to form transitive verbs with the general senses “to render, make” ( actualize; fossilize; sterilize; Americanize), “to convert into, give a specified character or form to” ( computerize; dramatize; itemize; motorize), “to subject to (as a process, sometimes named after its originator)” ( hospitalize; terrorize; galvanize; oxidize; simonize; winterize). Also formed with -ize, are a more heterogeneous group of verbs, usually intransitive, denoting a change of state ( crystallize), kinds or instances of behavior ( apologize; moralize; tyrannize), or activities ( economize; philosophize; theorize).
Don't get lost in the suffix jungle!
Compare reality, realization.