It depends on the topic.  In some areas and fields there either isn't enough evidence for a consensus or that evidence can be interpreted many ways(like economics), or the field is subjective to a degree to begin with (philosophy or theology).

Even within fields, it may depend on the actual topic.  For example, in the biology … Read more

Could be that we don't know everything! You might be the top expert in a field, but there are some things you don't know and you might be wrong. This applies to the scientists too. On the things they do not know, they go with what they think is the right answer.