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.
I have seen articles that track IQ of countries in Northern Hemisphere vs countries along the equator. The IQ of people in colder countries is significantly higher than along the equator. Maybe historically people who lived in cold climates had to struggle to survive against the climate and this caused them to exercise larger % of the brain in order to survive??
I recommend Richard Feynman's writings. Read about his role in solving the messy problem of the Challenger Disaster. Feynman did not trust complex methods with hidden assumptions Instead he started with basic physics and science and derived hypotheses and conclusions.
A second scientists whose method I think is under-appreciated is Darwin. His simple methods came to conclusions from very messy data. He is noted for exactness and attention to details and creating the science of evolution.
LOL - "When will [country] develop the nuclear bomb?" - I would ask the CIA on this question. One technique they would use is to monitor what type and quantity of equipment and raw materials they are importing. Of course they(CIA) monitor testing of underground explosives.
I recommend Tim Harford "The Logic of Life" "The Underground Economists" He illustrated that many questions can be answered by investigating it from unexpected directions. His speciality is economics.
Two of his questions: Why do some neighborhoods thrive and others become ghettos? Why is racism so persistent? Why is your idiot boss paid a fortune for sitting behind a mahogany altar?
If you take Tim Harford as an example then look for other people that think outside of the box and answer tough question.
I would not trust very deep and complex methods that cannot be explained in detail at a level the user can understand. A user should gain experience with any system or method before trusting it.
I enjoyed you question - hope this helps.
In terms of countries near equator are you seeing cause or effect. Equator regions should have been able to live off the land (eat and use plants that they did not have to grow) In northern countries people either hunted to survive or became agriculture based society. Either way they would have put in more planning and effort. Repeat this for a few hundred years might result in more creativity and reasoning ability.