The names are in flux i.e. changed some places, others not. But maybe you missed seeing
https://meta.discourse.org/t/what-do-user-trust-levels-do/4924/6?u=mittineague
Unlike other trust levels, you can lose trust level 3 status. If you dip below these requirements, whether from flagging or the passage of time, you will be demoted to Regular. However, in order to avoid constant promotion/demotion situations, there is a 2-week grace period immediately after gaining Trust Level 3 during which you will not be demoted.
A bit confusing Regular will be demoted to Regular, but you get the idea.
- also not changed yet at Discourse Meta