One is that it's one question. Just one question to summarize your life.
The second is that the ends of the scale has always seemed sort of odd to me: "he top of the ladder represents the best possible life for you and the bottom of the ladder represents the worst possible life for you."
Someone who is depressed is probably going to shift a bit in their evaluations of the top and bottom. That is, someone who is visibly and diagnosably depressed will probably imagine a worse best possible life, and also maybe even worse worst possible life. So you could be pretty darn depressed and still rate yourself as moderate. If your options go from "depressing" to "unmentionable hellscape" you could still rate yourself as a 7. Are most people doing this? Maybe not, but it's an odd way to frame the question and leads itself to that.
The final concern I have is how many of these are on the phone. If someone randomly called me on the phone to ask, would I go there even if I was really depressed? Probably not.
I guess I see these results as pretty interesting but really difficult to interpret. It's like trying to gauge global ocean temperature changes from one temperature reading in your local bay.