Schiit Yggdrasil -- 21 bit?


Schiit says that Yggdrasil is a 21 bit DAC. But the DAC chips that they put in the device ( Analog Devices AD5791BRUZ, 2 per channel) are 20 bit with the error of plus-minus 0.5 LSB.

How can the DAC be 21 bit if the chips are 20 bit? Using two chips per channel does reduce the RMS voltage of the noise by  a square root of 2. But how can you get to 21 bit from there?

Can someone please explain.
defiantboomerang
You are not against independent reviews, are you?

Of course not.

Your original question is valid, however, it hadn't really been answered (accurately or inaccurately) until after my post, when shadorne provided additional clarification and almarg offered yet another possible explanation.
@gdhal 

Sure thing. Well, I have learned a lot from some of the answers. I think that Schiit took a courageous step to use R2R chips in their DACs. They have one groovy machine there.
Putting one chip on -ve and one chip on +ve of balanced will simply double the voltage but won’t increase the bit depth. There really is no way to increase bit depth without a DAC chip with greater bit depth so I think Schiit is stretching the truth. I think that they are trying to say there is still signal below 20 bit due to dithering (random noise) that they must add prior to bit truncation of the last 4 bits of the 24 bit input signal. If they don't have the processing power to do dithering (normally done on a computer) then the audio quality will be compromised.