Some say that "true mint" (or a true "C10") is something that exists more in theory than in reality. Even if the toy has never been resealed (which may be more likely with MOSC figures), it's virtually guaranteed that the packaging will suffer minor imperfections just from being handled by factory workers, being packed into cases, shipping*, being shelved**, having price stickers attached***, being handled by prospective shoppers****, et al.

While toy grading is a highly subjective thing, it's nigh impossible to find a toy that's truly perfect mint or C10, because there's bound to be some micro-flaws in the toy, ranging from tiny imperfections in the packaging (e.g. nicks, scratches, dents etc. on the cardboard or plastic), or even manufacturing imperfections on the toy (e.g. tiny imperfection on paint apps, which seems to be occuring more frequently ), etc. What most people would imagine as C10 would be more like C9 or C8 i.e. a few notches down from "perfection." Basically, C10 is "perfect mint"... and there's no such thing as absolute perfection (or at least, it's highly unlikely).

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*The arduous journey from factory to warehouse to backroom
**Not every staff member is going to delicately remove toys from cases and ever-so-carefully place them on pegs and shelves. I've sometimes seen toys squeezed into shelves that are too short for the packaging with the card bent to force it to fit.
***Leaves glue residue even if you carefully remove it; some collectors try to remove it by carefully swabbing the residue w/ alcohol wipes. Some stores (like TRU) attach those those big security tags which dig into the packaging too.
****Or being bumped off by accident, or even maliciously thrown about by bratty kids.