Opposite-phase binaural signals stay louder than in-phase ones up to 40 dB above threshold

Evidence

The results at low signal-to-noise ratios indicate - as expected from BMLD data - that presenting the 250-Hz tones binaurally in opposite phase produces greater loudness than in-phase presentation. It is astonishing, however, that this increment is effective not only near masked threshold but up to 40 dB above it. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1728

Categorization

• QUESTION: What mechanism keeps a masking-level-difference effect going 40 dB above threshold where masking should be irrelevant?
• corroborated: 2 independent passages support this claim — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1728