The dB levels of REW are NOT calibrated. You need to find out at which level the input clips and then dial the source 3 dB down.
The Gustard uses 2 DAC chips, the E30 one.
Besides, after the chips there are opamps with resistors. The latter have tolerances.
These tolerances and component differences can easily become 0.2dB (about 2%) which I think is on the high side for electronics.
I haven't seen many headphones yet with a channel balance within 0.2dB over the entire audible band.
Yes, on AB you can hear 0.5dB differences. (I can detect them too) but not 0.2% using music. 0.1% I cannot detect, nor with tones nor with music.
About 30 years ago I built this device to check my (and those of others) hearing limits.