In the age when all PCs had 4 planes max, the Amiga supported 5 or 6 counting EHB mode. The AGA chipset was just an incremental kludge on that since the AAA chipset was taking too long. It was important in the beginning to support odd-numbered bits-per-pixel to get 32 or 8 colors without wasting RAM. By the time AGA came out, they just came out with a hack without even updating the blitter for fast-page mode much less chunky as well.
