why do dmix and dsnoop 's slave alway open subdevice 0 ?

for those hardware mixing sound cards which has two or more playback subdevices or hda-intel which may have 3 recording subdevices

when subdevice 0 is opened by another application ,

dmix / dsnoop always fail even when there are available subdevices

aplay -v -Dplug:dmix test.wav ALSA lib pcm_dmix.c:1010:(snd_pcm_dmix_open) unable to open slave aplay: main:654: audio open error: Device or resource busy

arecord -v -Dplug:dsnoop:1 -f cd -d20 tt.wav ALSA lib pcm_dsnoop.c:593:(snd_pcm_dsnoop_open) unable to open slave arecord: main:654: audio open error: Device or resource busy