Due to the number of actions that occur in Trello, nested action resources filter out specific action types. For example, changing the name of a checklistItem will trigger a webhook on the board and will POST an action of type updateCheckItem with the old and new name of the checklistItem. However, no nested actions resource will return action types of updateCheckItem: /cards/{id}/actions/, /boards/{id}/actions/. However you can get that specific action instance via GET/actions/{id}/.
We differentiate between the two types of actions as Included or Excluded. Included means that the action type will be returned when make a GET request that includes actions as a nested resource (eg: 1/members/me/actions). Excluded action types only appear in webhook data and are not returned in requests to nested resources.