[JohnM]

Can be the case, depending on what your setup is and what you are measuring. At very low signal levels noise contaminates the measurement, as the signal rises clear of the noise a truer picture of the harmonic levels emerges, as you start to approach saturation/clipping levels distortion components rise again (especially 3rd where clipping is involved). Bear in mind that approaching limits anywhere along the chain will cause distortion - i.e. the subwoofer input stage, the subwoofer amp, the subwoofer driver, the microphone, the mic preamp and the soundcard input. Can even introduce distortion with the test signal if its level is higher than the driven input can accommodate (unlikely with most sound cards though).