Lots of problems have more than one solution. Maybe telling you your answer was wrong was the test. Sometimes employers are looking for you to push back and say "Look. My answer is also correct and here's why..." or maybe they were looking for you to entertain the possibility of the interviewer's answer and consider that it may be the better way to go.

They're gauging your response to a certain stimulus. They want to see how you handle the situation because similar situations likely arise on a regular basis in their workplace and you'll have to be able to handle them effectively.

It seems to be a little-known fact among society but lots of problems have more than one solution. Maybe telling you your answer was wrong was the test. Sometimes employers are looking for you to push back and say "Look. My answer is also correct and here's why..." or maybe they were looking for you to entertain the possibility of the interviewer's answer and consider that it may be the better way to go.

They're gauging your response to a certain stimulus. They want to see how you handle the situation because similar situations likely arise on a regular basis in their workplace and you'll need to be able to handle them effectively. In many cases, this necessitates being an exceptional improv artist in order to handle unexpected situations with grace.

It just depends on what they're looking for and this is why it helps to research the employer as much as possible beforehand or have experience in the industry. If you have no idea what they're looking for it's nearly impossible to come up with an acceptable response. You need to ask yourself how somebody in this role would handle the situation and research/experience certainly helps you figure that out.