the 'gb' in the labelled capacity is 1,000,000,000 bytes. 1gb of actual nand capacity is 1,073,741,824 bytes. if you redo the math, the 300gb and 600gb models have about 13% spare area.
the 120gb model can't be 8 x 16gb, as the controller is 10-channel, so it's actually 5 x 8gb + 5 x 16gb (like its x25-m 120gb predecessor).
160gb and smaller models have ~7% spare area, like the x25-m.