[morzh]

That can be explained.
The plate detector works essentially as a tuned C-class amplifier.
While fairly linear at the top, it curves at the bottom and so with low amplitude signal the result is distorted as it is at the bottom of the curve.
However the whole range is not exactly linear and so the lower the modulation, the less it gets from the particular point at the curve so it gathers less distortion, as the smaller part of any curve one cuts out, the closer it is to a line.