Source

File: wp-admin/includes/class-wp-users-list-table.php

public function get_columns() {
		$c = array(
			'cb'       => '<input type="checkbox" />',
			'username' => __( 'Username' ),
			'name'     => __( 'Name' ),
			'email'    => __( 'Email' ),
			'role'     => __( 'Role' ),
			'posts'    => __( 'Posts' ),
		);

		if ( $this->is_site_users ) {
			unset( $c['posts'] );
		}

		return $c;
	}