return value of opus_dred_parse

for encoder i set OPUS_SET_DRED_DURATION to 100,
while decoding stream , i set dred_input with loss frames , but the return value of opus_dred_parse sometimes smaller than dred_input , sometimes larger !!!
i dont no why ?
how can i deal with the return value smaller than input ?

/* Only decode the amount we need to fill in the gap. */
ret = opus_dred_parse(dred_dec, dred, data, len, IMIN(48000, IMAX(0, dred_input)), sampling_rate, &dred_end, 0);
printf("--------dred_input=%d, opus_dred_parse ret = %d\n", dred_input, ret);

log print:
--------dred_input=960, opus_dred_parse ret = 720
--------dred_input=960, opus_dred_parse ret = 720
--------dred_input=960, opus_dred_parse ret = 720
--------dred_input=960, opus_dred_parse ret = 720
--------dred_input=4800, opus_dred_parse ret = 4560
--------dred_input=960, opus_dred_parse ret = 720
--------dred_input=1920, opus_dred_parse ret = 2640

Start at the opus_dred_parse entry point and reproduce the reported calls using OPUS_SET_DRED_DURATION set to 100 and the logged dred_input values. Compare the return value with the supplied input and sampling rate to establish the API's expected behavior. Done means the differing results are explained and the correct handling or required project change is identified.