Error pagination di codeigniter selalu mulai dari angka akhir

Controller

public function siswa($nisn)
    {
        $config['base_url'] = base_url().'view/payment/siswa/'.$nisn.'/index';
        $config['total_rows'] = count($this->pm->hitung($nisn));
        $config['per_page'] = 6;

        $this->pagination->initialize($config);
        $data['page'] = $this->uri->segment(6);
        $data['data_siswa'] = $this->pm->ambil_data($nisn,$config["per_page"], $data['page']);
        $data['nisn'] = $nisn;
        $data['pagination'] = $this->pagination->create_links();
        $this->load->view('payment', $data);
    }

Model:

public function ambil_data($nisn,$limit,$start)
    {
        $this->db->select('*');
        $this->db->from('siswa');
        //join LEFT by default
        $this->db->join('pembayaran', 'pembayaran.id_siswa=siswa.nisn');
        $this->db->join('tipe_pembayaran', 'tipe_pembayaran.id_tipe_pembayaran=pembayaran.tipe_pembayaran');
        $this->db->where('siswa.nisn = '.$nisn);
        $this->db->order_by('tipe_pembayaran','ASEC');
        $this->db->limit($limit,$start);
        $query = $this->db->get();
        if($query->num_rows()>0)
        {
            foreach ($query->result() as $data) {
                $hasil[] = $data;
            }
            return $hasil;
        }else{
            return FALSE;
        }

    }

    public function hitung($nisn)
    {
        return $this->db->get_where('pembayaran', array('id_siswa' => $nisn))->result();
    }

pagination selalu mulai dari angka terakhir.