I figured out an unexpectable behaviour of posts lists:
For printing out a posts list by /wp-admin/edit.php you can add new columns via the hooks 'manage_posts_columns' and 'manage_posts_custom_columns'.
Assume the post type is not 'post' but a custom post type, e.g. 'portfolio'. So you can print out a list of portfolio posts via /wp-admin/edit.php?post_type=portfolio.
I would expect that the formerly mentioned hooks do not work here, only the hooks for that custom post type 'manage_portfolio_posts_columns' and 'manage_portfolio_posts_custom_columns'.
But it seems that added functions via 'manage_posts_columns' and 'manage_posts_custom_columns' are also working in the portfoliio list.
Why that? And how to let functions only for the posts list no work in the portfolio list ?