For the record, I am NOT a day-to-day Lisp programmer, but I've played with it a little and read about it some.
I think it's because its about expressing your problem clearly. It's not "20 times less lines" so much as it is "20 times clearer expression". Not that it's exactly 20x, but it seems better. When you express things more clearly, and structure your code more like your problem, and less like your programming language, you shed a lot of accidental complexity ( See Out of the Tarpit - http://shaffner.us/cs/papers/tarpit.pdf ), so managing the complexity of your project is easier, which means fixing bugs and adding new features is also easier. I think this is the route being implied when they say you can develop features faster. I'm not so certain you'd see the benefit in a particular feature, but over time.
Note that a lot of the modern languages are picking up many of these expressive features, closing the gap there as well, which is wonderful.
You can, simplified, see Lisp as the grandfathers of the scripting languages. (But with parse trees as syntax, efficient compiler decades ago and so on.)
The point with the scripting languages is that development speed is so much quicker than e.g. C, for many cases. Lots of less code is needed. Personally, I went to scripting languages from C originally because of that the sheer productivity is so fun. I would have gone Lisp if there were jobs.