My assumption was the box was to replicate the space within the wall studs including between the blocking. So if you have 16-inch spacing on a 9-foot ceiling height with blocking half-way up, you'd be left with a space 16-inches wide by the width of the framing material (about 3.5-inches here in the USA) over the 4.5-foot (54-inch) height of the cavity. Would come to just under two-cubic-feet. That's about what they're showing.