The difference is that in the google home app it shows playing. In HA it shows off.
The problem here is that you can’t adjust the volume for an individual player in the group.
In Google Home app you can adjust the volume synchronized for all players just like in HA, but you can also adjust the volume individual. Because the players is in off-state HA can’t adjust the volume.
I have an automation looking like this:
action:
service: media_player.volume_set
entity_id: media_player.vaggen
data_template:
volume_level: '{{ states.media_player.vaggen.attributes.volume_level + 0.05 }}'
This won’t work because the state of that player is off when using multiroom.