My understanding of the GA entity hierarchy model is the the profile entity is the parent of the experiment entity. If you go to the content experiment dashboard, click the site usage tab in the reporting. Is the % new sessions column referring to the % of sessions that are new to the experiment or are they the % of sessions that come from new users. I am doing advanced segments "new users" and the numbers do quite line up but they also don't seem different enough to be explained away completely.
% new sessoins refers to an estimate of the percentage of first-time visits. You can basically understand this as the sessions that are created by new users.
Google Analytics provides the basic terminology on the titles, i.e. you can hover over a title and a help text will appear which shows the meaning of that term.
It is an estimated percentage of first time visits. It counts new users so by definition it excludes returning visitors. In example a returning visitor that is experiencing his second session will not be measured in %new sessions