It means going the extra mile needed to achieve internal logical consistency.
The ocean is filled with lots of contaminants, such as plastics, oil, and extra carbon.
The great food that summer ended up putting 25 extra pounds on my frame.
1650s as a stand-alone adjective; also used as an adverb and noun in 17c. (see extra-); modern usages -- including sense of "minor performer in a play" (1777) and "special edition of a newspaper" (1793) -- all probably are from shortenings of extraordinary, which was used extensively in 18c. as noun and adverb in places we would use extra today.
only recorded in classical Latin in extraordinarius, but much used in Medieval Latin and modern formations; it represents Latin extra (adv.) "on the outside, without, except," the old fem. ablative singular of exterus "outward, outside," comparative of ex "out of" (see ex-).
extra- or extro-
Outside; beyond: extracellular.
Object-oriented, Pascal style, handles sets. "A Data Model and Query Language for EXODUS", M.J. Carey et al, SIGMOD 88 Conf Proc, pp.413- 423, ACM SIGMOD Record 17:3 (Sept 1988).