[Drupal] [development] Drupal 7 Taxonomy CCK question

Hello,

I have created some terms (vocab=test) in Taxonomy and linked them
with a content type CCK using "Term reference".

I found they are linked up in the DB by the table "taxonomy_index" and
"field_data_field_test", why duplicated linkage (both table contains
some data)?

Seem taxonomy_index is not needed and data inside not incorrect, right?

Is it a bug?

Thanks.