It is also somewhat unlikely that exploration would be conducted so that each new survey project would add a beautifully symmetric "curved cube" to the volume known. Those cubes probably represent volumes where some exploration has been conducted, while space outside the cubes represents volume where no exploration has ever been conducted.
When we see a cube on the edge of the explored space, that particular cube may be only 0.00002% explored, while a cube close to the center may be 57% explored already. It's unlikely that any of the cubes would be 100% explored: see e.g. "Who Mourns for Adonais" where Pollux, a system quite proximal to Earth, harbors secrets so big that it cannot be considered explored in any practical sense. Not by the UFP, that is.