rhyme
The 20 to 22 must refer to acceptance, not yield. A 20% yield rate would imply that they would make nearly 3000 offers to fill a class - with an app volume likely comparable to Kellogg's that would place their admit rate at something like 70% - and if you figure half the applicants who apply dont get an interview, then the post-interview admission rate would be pegged at 100%, and they would still fall short of filling each slot.
I believe jb fixed his post to reflect that. It's definitely acceptance rate.
However, the fact still remains that if there is a higher than expected yield for R1 offers (and they don't withdraw), then there will be fewer spots available for R2 applicants. This will likely lead to a higher WL/Ding percentage for R2.
I really need to stop reading this board for the next few weeks until calls start coming in ... it's getting more nerve wracking by the hour!