Comparing two tones improves frequency sensitivity beyond detecting a continuous modulation

Evidence

The direction of this difference is astonishing: our hearing system is more sensitive to frequency changes if the task is to recognize differences rather than to recognize modulations. The pause between the two sounds to be compared does not reduce the sensitivity, on the contrary, it increases it! Displacing the data given in Fig. 7.8 down by a factor of three produces a reasonable approximation to the results at the two asymptotes: i.e. at frequencies below 500 Hz, we are able to differentiate between two tone bursts with a frequency difference of only about 1 Hz; above 500 Hz, this value increases in proportion to frequency and is approximately 0.002/. — Professor Dr.-Ing. Hugo Fastl, Professor Dr.-Ing. Eberhard, Psychoacoustics- Facts and Models, 1069

Categorization

• QUESTION: Why does a time gap between compared tones increase rather than harm frequency sensitivity? The memory-limitation explanation for low modulation rates seems to conflict.