Freezes are just deadlines that mark partials milestones in the road map to get the final release. Ej.
Feature Freeze: At this point we stop introducing new features, packages, and APIs, and concentrate on fixing bugs in the development release.
User Interface Freeze: The user interface must be stabilized at some point, so that documentation writers and translators can work on a fixed target that doesn't obsolete screenshots or documentation.
Here you can get cd images that even not finals, meet those partial milestones. I mean, if you retrieve a cd image of the daily build March 13th you must expect a system that will not have changes in UI compared with the final release, apart the existence of some exceptions.