When editing a field group ACF loads the field group (post) then loads the fields (child posts) for the group. The field keys are part of the post and when editing a field group everything is dependent on the field key.
When editing content, again, all editing is done by field key.
The field name is really just a convenience for us to make it easier when coding or if you want to use a function like get_post_meta() to get the field value.
Yes, if a value is save to a field for a post there will be an entry in the DB that relates that field to the post, but this is so that ACF can determine the unique field object since field names do not need to be unique.
