That's because A, B, C not because they have any idea about an equation.
"Have you heard of the pythagorean theorem?"
"Have you heard of E=mc^2?"
Granted, one is the equation itself and one is the name - but that is how someone without interest will know both of them legitimately. If you ask them A + B = ? They will say C as well usually. They have no idea what they are saying they are just using their common sense and applying their alphabet where there appears to be letters.
Just because someone has been through school doesn't mean they will remember it either. It's a safe guess that 80%+ of the people who have heard of E=mc^2 couldn't even explain what the equation stands for, never mind what it means. More people will understand the pythagorean theorem since it is a fairly basic math concept.