The jumpers represent different powers of 2 and they're additive. Hence, 0 => 2^0 =1; 1 => 2^1 = 2; 2 => 2^2 = 4.
For example, jumpers on 0 and 2 gives you 1 + 4 = 5, so SCSI ID 5.
Jumpers on 0, 1 and 2 would give you 1 + 2 + 4 = 7.
Some devices actually label the jumpers 1, 2, and 4, instead of 0, 1, and 2, which is a little easier to understand.
