Done counts steps that are FINISHED — done and skipped alike, since a step the org deliberately passed over is not still owed. It therefore rises when somebody skips, which is the intended reading of a checklist.
Next is the id of the step to do next: the first available, unfinished step in authoring order. Empty when the journey is complete, and also empty when every remaining step is blocked by a dependency.
Total is how many steps this org's journey holds — the ENABLED steps of the playbook, so it shrinks when an operator disables one and does not match the authored step count.