Example make.conf: USE="foo" USE="${USE} bar" will be parsed to show that USE="foo bar" was set in make.conf second example make.conf: USE="${USE} bar" this second example will be parsed to show that USE="${profile_use} bar" was set in make.conf which causes the USE setting in make.defaults to then override all other settings such as those in package.use
I guess it makes sense to treat incremental variables as unset at the beginning of a getconfig call (which may parse multiple files if a directory is used in place of a file). For regular non-incremental variables, it makes sense to be able to access/substitute values set earlier in the profile, but for incremental variables it doesn't seem to have any valid use case.
*** This bug has been marked as a duplicate of bug 771549 ***