The answers here are pretty good but I did want to offer an alternative from my experience that could explain things. I have worked for a couple of IT consulting firms that appeared, from the outside, to have strange interviewing and hiring tactics. These tactics make more sense once you are on the inside.
The first company never ever had a concrete open position to fill.. They subscribed to the "Hire really smart people and let them do their thing" mantra.. The business model was to get clients to pay for consultants by the hour so it was never "we need a web developer" it was "can we justify billing over $200 an hour for this person"... If the answer is yes, welcome to the team, if no, we dont need to create a position for this person. We would let just about anyone into the hiring process since the only requirement of "getting the job" was to impress the hell out of everyone in the interview.
The second company was similar except that they would get a request from a client and then scramble to fill the position. I interviewed with both the consulting firm and then the client. All went well except when I got the call back, the consulting firm informed me that their client was pulling out of the deal for an unrelated reason. The "open position" vaporized instantly. I incurred some expenses since this firm was in another city but I handled it well... they called back a year later and gave me a spot at a different client with no second interview.
I agree that this kind of example is pretty rare but also, I think, an understandable situation depending on industry and business type.