Field is translatable, but field instance is not
Every node field has one storage setting. Here the field is configured how it is stored in the database, field type, single or multi value, or translatable or not. This settings can't be changed after the field is set up and filled with data. It's shared among all content types.

The field can be used in multiple content types. In each content type it can be configured to be translatable. (Option "Users may translate this field"). This setting can be changed any time and without data loss.

If you want the field to be translatable, you need both options set, in the field storage and in the content type where the field is used.

If you use the UI, then you don't have to worry about this, because Drupal sets the storage settings the first time you place a field in a content type.