Ah, but I'm not sure that "a single set of life experiences" applies in the reported cases in which one (or more) personality(s) apparently arise/are split off to "shield" the rest from the experience and memories of abuse. The classic (and possibly incorrect and/or fictional) scenario, as described in a comment further down the page, is a protector persona which remembers things that the protected part of the mind doesn't (isn't allowed to, represses, etc) for its own good and/or relative sanity.

tl;dr - "I can't deal with ____, so I shall imagine someone who can and let them remember it while I forget."