People are more complicated than machines. First, imagine a condition like "high blood pressure" or "underactive thyroid." It's not just a case of measuring one number about you and comparing it to one "correct" number. There is a normal range for healthy people. Imagine some measurement that for normal people is between 10 and 12. Imagine that you genuinely have a shortfall in this number; and you vary between 8 and 10. One doctor sees you on the day you measure 10 and tells you that you're fine. Another doctor sees you on the day you measure 8 and tells you there is a shortfall. So that's one explanation.
But life is actually more complicated than that. Some doctors might think 9-13 is actually normal for that number, so if you measure 9 they'll say you're fine. Others might think that it depends on some other reading, so if you measure 10 but the other number is low, they'll say you're ill, while another will insist that a reading of 10 means you're fine. And doctors vary on how urgent they think certain conditions are: some think they can advise you to change your diet or to exercise, while others want to start you on medications, even when they both agree you have a particular condition.
Then on top of all of that there's the matter of how the doctors get paid. Some want to recommend particular treatments or tests as much to ensure an income for themselves as to actually fix you. We of course all hope that such people are few and far between, but they do exist, and you may have met a few of them in your time, or your relatives may have.
How should you behave? You have two choices. One is to focus on your symptoms: you're tired, or your eye hurts, or whatever, and ask your doctor to help you learn why and help you fix that so you feel better. If they do a measurement and say that it's fine, you ask them to do some other measurement or test so that they can suggest things to do that will help with your symptom. The other approach is to learn more about the condition you think you may have. Say you think you have high blood pressure. Learn what a normal range for each number is. When your doctor measures yours, ask what the measurement was. Learn the things you can do - how you sit, for example, that can affect the measurement. Ask the doctor's conclusion about the measurement - is it high? is it fine? - and if your conclusion is different, ask why. Why is that high? Don't people normally vary between x and y over the course of a day? Or why is that fine? Learn some of the thought processes your doctor is going through.
Getting a second opinion is a fine strategy. But "Dr A says I need a medication and Dr B says I do not" is nowhere near as valuable as "Dr A measured 8 and says normal is 10-12 so I need this medication to raise it, but Dr B measured 10 and says normal is 9-13 so I don't." You can now go and learn more about what a normal range is, the consequences of being low, the side effects of the medication, and so on, then make an informed decision, including which doctor you want to work with over time to maintain your health.
