1760, "process of calculating a sum," from Modern Latin summationem (nominative summatio) "an adding up," noun of action from Late Latin summatus, past participle of summare "to sum up," from Latin summa (see sum (n.)). Meaning "a summing up" is from 1836.
word-forming element making adjectives from verbs, meaning "pertaining to, tending to; doing, serving to do," in some cases from Old French -if, but usually directly from Latin adjectival suffix -ivus (source also of Italian and Spanish -ivo). In some words borrowed from French at an early date it has been reduced to -y (as in hasty, tardy).
Others are reading
Definitions of summative from WordNet
of or relating to a summation or produced by summation;