@Mr
What does it label 5 and 7 as?

with your cheapo multimeter what resistance does it measure when you hold the meter leads together? Do you have any jump leads?

My guess is your measurement is way out. try bridging from batt -ve to clean metal near the starter?

Did you do a resistance measurement from battery positive to the starter?

Simple explanation of voltages and resistances with apologies to those for whom this is obvious, lets say for example the lead to chassis is 0.1 ohm (that would be high) and the starter motor is 0.05 ohm, total resistance combined is 0.15 ohm.

ohms law V=IR or I=V/R or R=V/I

Current through both, I is V/R=12/0.15 = 80A

Voltage across earth strap =IR = 80x0.1=8V (wattage dissipated in strap 640 watts)

Voltage across starter =IR = 80x 0.05=4V (wattage dissipated in starter 320 watts)

total volts =8+4=12

Does that help? (it does illustrate why the connections to earth and starter need to be very low resistance)