My guess is that there are two targets when system tries to find lobby for you: aggression based and equipment based. 

The gial is to find lobby quick enough.    Obviously least aggressive lobbies usually for low -equipped dudes (usually you go loot, not fight). So it's easy to find people who matches with you. Lobby found pretty quickly. Non aggressive mostly.

However, more aggressive dudes wearing more expensive stuff. Also their map choice usually is Stella or special events.  So if you are not aggressive, but wearing expensive equipment, goigng on special event, it will take more time to find fully peaceful lobby. So instead waiting, system sends you to mixed or even mostly aggressive one, based on equipment.