delete_post_meta only removes 5 most recent entries

I am creating an uninstall file for a small WordPress plugin i wrote.
I want to remove the entries I added in the wp_postmeta table.
I use the following code for it as describet in the codex:

$allposts = get_posts('numberposts=0&post_type=post&post_status=');
foreach( $allposts as $postinfo) {
	delete_post_meta($postinfo->ID, '_myPluginName');
}

But it only deletes the entries from the 5 most recent posts (the other data stays just in the database).

When I ‘prinr()’ the $allposts, I also get only the data from the 5 most recent post.

How can I fix it?