Ok I undestand this quite well now. Now suppose we're on ground and we have a 20 knot tailwind and stationary. I'll try and repeat exactly what you did, just correct me where I'm wrong:
The TASmeter and Airspeed Indicator would show zero as there is no -20 knot value, the GROUNDSPEEDmeter would also read zero and the anemometer on the ground would read 20 knots.
If we accelerate to 180 knots relative to the ground, the TASmeter and Airspeed Indicator would read 160 knots, and the GROUNDSPEEDmeter would read 180 knots. The anemometer on the ground would remain unchanged as we suppose that the wind hasn't changed.