Du bist ein Student? Das hätte ich nicht gedacht.
You are a student? I never thought you were a student.
It did not came to my mind you could be a student. I wasn't wondering about it. My thinking is purely hypothetical.
If you cannot wrap your mind around this, think counterfactual instead of hypothetical.
I did not think about you, though I just told you I had.
That's what the Konjunktiv does. It turns facts into counterfacts.