OK, I think I'm going to try this variant out on my large, high-level group of PCs next session. Hopefully it will make things a little easier on me during one of our big combat encounters.
After double-checking Unearthed Arcana, it states pretty clearly that a defense check that meets or beats the attacker's DC is a miss. Which means that the 11 is an error, and the DC should be calculated using 12 + the enemy's attack bonus.
I'm surprised I haven't seen more people point out this glitch.