DAC+ADC PRO input level halved when MONO

While trying to characterize Adc input/PGA, it turned out that signal level of mono recording is half the level recorded in stereo.
I sounds weird, is it normal ?

Experiment settings:
DAC+ADC Pro, signal generator at +/- 100mV sinus 1KHz, PGA gain at 20dB on both channels (amixer -c 0 sset ADC 20dB, checked in alsamixer), wav signal level +/- 0.353 in stereo, +/- 0.176 in mono. 
Recording using Audacity: 192KHz, chosen input snd_rpi_hifiberry_dacplusadcpro...(or sysdefault).

Using arecord in a terminal, changing sampling Fq or signal frequency does not change the result

Stereo signal gain is conform to theory for this PGA gain (less than 1% error) i.e. wav amplitude=input in volt*PGA (10x here)/(2*squareroot(2))


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