because the only way to discover anything more from the green tile below the 4 is to have it be the minimum value, and using uniqueness logic to disprove other uniqueness logic assertations is invalid as it leads to multiple correct solutions (by assuming the lower-right green tile is safe first, you can "disprove" the tile to its left, but you can also make the correct deduction that both are safe through the above logic. both solutions are valid if you allow one uniqueness logic leap to disprove another)