Arrive slow and the sink rate at the threshold is already above schedule, the flare is short because there is no energy to extend it, and the touchdown is hard in a way that feels exactly like a late flare. Diagnosing a slow arrival as a late flare is the catastrophic misread: you respond by flaring earlier, which bleeds more speed, which makes the next one worse. The tiebreaker is threshold speed against VRef, not feel. Never diagnose a flare without it.