The Subpsace Rebounder was found as a kind of fluke. While researching Shields III, one of our most brilliant scientists, who happens to be phsyically disabled, came up with the idea of an energy field that rebounds energy into a slightly different angle that allows our shield technology to have to absorb less of the blast.
In effect, it causes energy weapons to nearly miss. It is more expensive, by far, because it still requires the full cost of including shields, but it doesn't use any more space.