Use the slider to make the two shapes (or brightnesses) look the same. Even after checking "Show answer" or hitting "Auto-match to equal," so the values are genuinely equal, they may still look different to your eye.
Would you believe it if two lines are exactly the same length when you measure them with a ruler, yet your eyes keep seeing them as different? This kind of phenomenon is called an optical illusion. It's not that your eyes are broken — it's a natural result of the way your brain interprets what it sees.
One of the most famous illusions is the Müller-Lyer illusion. Place a line with outward-facing arrowheads next to one with inward-facing arrowheads, and the outward-facing one looks noticeably longer, even though the actual lengths can be identical. The surest way to deal with an illusion is not to trust the impression your eyes give you, but to actually measure it. Use the slider to make the two shapes look the same, then compare against the real values to check whether they truly are.