Speculation: 7 bits are required to represent 0..100, and 7 is Just Weird, so you add an 8th to be able to do -100..100 in one addressable unit. (Or use the 8th

Speculation: 7 bits are required to represent 0..100, and 7 is Just Weird, so you add an 8th to be able to do -100..100 in one addressable unit. (Or use the 8th bit to tag a variable-length numeric value.) 4 bits for 0..9 as in BCD is probably more convincing.