Asked questions are often variations of common questions, where the asker just doesn't quite know what the problem is called, so they can't find duplicates on the site, but there are really thousands of questions that can be answered almost the same way. Those questions are usually answered very quickly, because we are used to answering their variations.
Other questions that are answered very quickly are usually coding problems in a language that the answerer is very experienced in. When the answerer can read code like their first language it doesn't take them long to find the errors in a small sample of code.
Some questions don't get answered so fast. Those questions are usually more difficult to answer, and take more effort. Since many of the people who enjoy answering questions on SO are the kind of people who enjoy challenges, they will put the effort in to try to be the first to figure out an answer to the problem, so there are usually a few people at any given time trying to find the answer to any recent difficult questions. So for their difficulty, they'll usually be answered fairly quick too, unless they're specific to a less widely used technology.