During a visit to my family doctor, I’ve noticed something very familiar. They would ask me how I’m doing, what’s bothering me, and then I would tell them symptoms. I had pain when doing this and that. I had trouble sleeping after drinking this or that. And so on.
And then the doctor started to tell me what they thought was not my problem. It’s not arthritis because otherwise you would feel this. It’s not meningitis because otherwise we would see these and these symptoms.
That’s exactly how software people debug problems together with users. “What did you do here exactly? And then? What operating system and browser version do you run? What time of the day was it? Did it happen on the next day? How much data did you put in?”
And then engineers throw away certain less probable scenarios until hopefully reaching to just a few promising candidates for being a culprit of a bug in their system.