Imagine how you will feel two years after the death of your oldest child.
As soon as I read that sentence I considered the implications of the two years, how the time would have helped soften the loss, how I might have adjusted at some level to the loss by then. By contrast, all of the experimental subjects said "what? Are you kidding? I'd be devastated!" As if the death had occurred that day.
Similarly he notes how many people will commit to a future action without "seeing" the details of that action. Like saying sure, I'll babysit next month, having warm and fuzzy thoughts about the concept of babysitting without any thoughts about the minute-by-minute details. I am very careful about this type committment for that very reason, that I do think of the details. I put myself into that day and think how my life will be affected. It isn't abstract to me.
I suspect I am unusual in this sense, and I suspect this is so because I have consciously focused on seeing things clearly, whether they are right now or way off later. What will it mean if I take this class in throwing pottery? I will be getting up that morning, thinking about the time I will spend at the class, about how I might ask questions of the teacher, how much time I will spend in the car getting there and coming home, and so forth. It may be that I have gotten rather too good at this because I tend to refuse a lot of opportunities!