Here's a situation I've seen dozens of times: a friend interviews at a company. When I ask them how it went, they say: "I answered all of the questions, so I think I will get the offer." A few days later, reality kicks in - they didn't get the offer. In some instances, I've had friends tell me they screwed up in an interview (or two), and they still got the offer. What's happening here is that some people believe that an interview is simply about getting the "right" answers. In reality, the company conducting the interview is looking at many additional factors. I'll go over them below.
Most of these observations come from my background in Product Management/Technology, but I believe they apply in a broader environment as well.
Hiring the best candidate
A team/company hiring an individual is simply looking for the most qualified candidate for a given role. What that means is that the person who gets the offer is the one who performed better than everyone else who was interviewed (according to the criteria set by the hiring company). So if two candidates answer all the questions "correctly", still one of them won't get an offer. The deciding factor will be something else.
Note: I say "correctly" in quotes, because some of the better questions out there don't really have a single, correct answer - there could be many solid answers. For example: "How would you design product X?", "How would you deal with a customer who is yelling at you in public?", "What is something you're really proud of?"
Prior experience can be an important tie-breaker in situations like the one described above - two (or more) candidates giving overall good answers to all of the questions. If one of them has worked in a similar field before or can bring insights from a competitor, that candidate may have an edge.
The personality of the candidate and how it fits within the team/company is a very important factor. Hiring managers are often picturing the candidate coming to work every day - will they add value to the team? Will they get along well with others? Is this someone I want interacting with my other employees/clients/customers?
Some teams prefer a quiet person who will get work done and not cause trouble. Others need outgoing personalities who are not afraid to speak their minds. Some teams want purists, others want unorthodox people. Some want individuals who separate work from their personal lives, others like candidates who will make work their priority in life.
For a candidate, it's not always obvious what a company is looking for, but the interview panel will give you an idea of the people who already work there. If as a candidate, you feel a level of chemistry during interviews, chances are the hiring team will see a cultural fit.
When an interviewer asks a question, they're often looking for the process, ideas, and articulation level of the candidate. Reaching the "correct" answer is not the main challenge - it's being able to demonstrate confidence in how you arrived and express your conclusions, and whether people working with you would have an easy time communicating with you.
Some candidates will come off as arrogant/over-confident. Others will come off as shy. Some will be monotone, while others can't contain their passion and energy. The hiring manager will be looking for the person whose communication style fits better with the role at hand.
At the end of the day, hiring decisions are made by humans. So there will always be subjective factors that play a role. Maybe the candidate went to a rival school. Maybe they are under- or over-dressed. Maybe they have a mannerism that annoys the interviewer. Maybe the interview panel is in a bad mood that day. These are all factors that may hurt a candidate without them realizing it.
In order to ace a set of interviews, it's not enough to answer all the questions satisfactorily. The deciding factor is whether the candidate performed better than all other candidates, including aspects such as their experience, communication style, and cultural fit. There are always subjective factors that a candidate cannot control and could result in a negative outcome.
If you're interested in interviewing for Product Manager positions, I can help you.