The thumbwheel controls 4 bits (numbered 0 through 3). The hard drive only uses the first 3 bits to set the SCSI ID. The thumbwheel switches used on these drives still register a valid binary output even for the settings that are blank (they represent 8 and 9, for the curious).
Because the SCSI ID is set by bits 0, 1, and 2 (bit 3 is ignored), the two blank positions still represent valid (although useless and duplicate) SCSI ID's. See chart below.
Thumbwheel settings and their corresponding bit positions:
Thumbwheel Binary value
Setting Bits: 3 2 1 0
6 0,1,1,0 <--- SCSI address 6
7 0,1,1,1 <--- Obvious conflict with the CPU
8 1,0,0,0 <--- First blank position
9 1,0,0,1 <--- Second blank position
As the chart shows, thumbwheel setting 8 would equal SCSI ID 0 and 9 would be ID 1. To avoid confusion it is best not to set the SCSI ID switch to a blank value.