It depends on the job. What's the job?
If it's shoving stuff in boxes all day long(in a pre-defined order), they probably want to know they can trust you - you can tell alot about a person at an interview, and usually you particular answers don't matter.
If the job was something that required more skill and intelligence, it's easy to understand how they would want to see how you perform during the interview.
Perhaps the job requires people skills - during the interview, if you can't face up to a panel without shaking and 'umming and arring' ... well, you can probably see why they wouldn't employ you.
I once went for a telemarketing job, for example, that day I was tired, I was nervous, I couldn't think of what to say - even though I was ment to be talking about myself - and it was obvious that I couldn't do the job (however if I had of been feeling better, they would have seen a very different me)
Another example was a job that really required me to be able to socialize - and be honest. I did extremely well at that interiview. I didn't get the job, however I was told over the phone later that day, that I would be put on their list just incase there was anything coming up in the future.
I've done some interviewer skills training, and I realize that there is more to it than just getting all of the write answers filled out in the right boxes.
