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
@shadorne 

I understand many here do not care about measured performance as long as they like the sound.

Yes, I am becoming aware of that. Could one say that such measurements-don't-matter audiophiles are like junk food addicts? They like to eat what tastes good to them without caring about the ingredients in their food?
@defiantboomerang 

Yes but I would say most audiophiles are dining on delicious gastronomic meals full of flavor and expensive ingredients rather than junk food.

I am interested in the accurate reproduction of the source file or music as produced by the artist and their engineers/producer but I fully appreciate why others like things to sound the way they want. After all at a restaurant you often order the dishes that you like best not necessarily what the chef has specially prepared.

@shadorne 

Good point. Those people do order expensive ingredients. Using their methods of analysis they can’t really tell the good ingredients from bad ones, though. Whatever sounds good is good. Whatever tastes good is good. 
@defiantboomerang / @shadorne 

Frankly, I'm interested, care about, etc. both measured performance and the sound. But as I've mentioned and am experiencing first hand (not to mention second hand by listening to other systems), I already have four DACs in total, but discounting the USB DAC in my M6si, that leaves two delta sigma (one in ERC-3 another in UDP-205) and the multibit in the Yggy.  They all sound great and none of them play a fourth verse in a song that only has three. So should I be led to believe that there is in fact a "nirvana dac" that measures perfectly and sounds perfectly so I can hear this fourth verse?