In early COVID 19 infection some peoples natural resistance applies the "brakes"
to the virus. The data in the green circle is from patients with mild infection. Scientists measure the actual virus, not just the viral RNA.
As can be seen these "mild" cases indicate the person was able to slow, contain or put on the breaks, so the virus could not run rampant. In severe cases, the living virus load is far greater and hence able to wreak more damage to the lungs.