I can imagine that its based on the account trust and if its flagged as spam or not. People can report chats now so this might influence the inbox ratio.
To get 100% inbox ratio the people need to follow you. You can also send dms without following but chances are that some messages don’t go through.
Mass DMS has been patched by IG for awhile. Results from this method have been in a steady decline since last year. That’s why I personally use the mother/child method. my slaves follow very selective users and DMs them with an enticing message to bring attention to my Main account. Most DMs from non-followers generally don’t generate a notification.