I think your question is unclear because the question says How can people answer a question consistently? But it appears instead of learning how to answer messy questions yourself you want to know why others do not give consistent answers. Given diverse input information and different backgrounds I am not surprised that the answers vary. This page is a good example of the variability of answers.
A better plan is to understand the methods and reasoning used by the best experts in thinking have solved problems and answered questions. With this research you can develop criteria and methodology that lead to accurate solutions and consistent answers.