The devil is in the details of how you define those two terms. The shelters have a fairly wide latitude to decide, and the definitions cover a wide spectrum of different opinions.
In California, Monterey County Animal Control defines as "unadoptable" any animal for which it cannot find a home. Their shelters can establish their own standard for deciding when this is the case. On the other end of the spectrum, MaxFund, in Denver, Colorado, will not euthanize any animal except in cases of extreme physical suffering.
Bill Suro, co-founder and medical director of MaxFund, says the "adoptable" label is part of the problem. Shelters want to be designated as "no-kill" because a "no-kill" shelter attracts more donors.
On the other hand, there are too many abandoned animals, so many shelters give an "assess-a-pet" test. An animal that fails the test is "vicious" and "unadoptable." and the "no kill" shelter can euthanize it.
|Dog Being Given Sue Sternberg's Temperament Test|
Public opinion has been shifting toward "no-kill" for a couple of decades now. It seems possible that we may actually achieve a true "No Kill Nation" in the fairly near future.
But we aren't quite there yet.