The efficiency comes from the difference in electrical consumption by the two - the chest freezer doesn't "spill" the cold out like an upright freezer or refrigerator, so opening it to remove something isn't causing a surge of warm air to enter to displace the cold air that falls out of an upright device. Of course, if all other things are equal, it makes sense that unopened vessels that are equally well insulated will be the same efficiency, but I open my freezers and refrigerators relatively frequently - weekly, if not more, but I try to minimize openings whenever possible.