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
@georgehifi 

If playing CD quality files it really doesn't matter at all. Good point.

However in judging a product I do like to check manufacturers claims. For example technical claims by Hegel don't hold water. So either they got something messed up in translation from Norwegian or I have my doubts over technical competence.
I agree with @shadorne regarding (paraphrasing) drawing an inference to the product based on manufacturer technical claims. In fact, the boisterous claims by Schiit is one (of a few) reasons I purchased this particular DAC to begin with.

In addition to the boisterous claims by owners/reviewers :)
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Quite awhile ago Jason explained all this in his "Schiit Happened"
Book he has on line at Head Fi. Too long to go through to provide an exact link. They sold a ton of these and are still selling well.
Don’t like don’t buy it!

You got Atkinson saying it's obsolete on the other hand you got Harley saying it's as good as a $19k DAC's!
Buffoons all of them and that's why I don't subscribe even for $7 a year . They can't give Stereopile away.