The term of a patent is 20 years from the filing date. The legal status of the patent will be "expired" in the case of a patent filed in 1991,. This is because the term of the patent has been more than 20 years.
Further, once a patent is granted, it is must for a patentee to pay the prescribed renewal fees to keep the patent in force until the completion of the patent term. The procedure and rules for payment of the renewal fees varies from jurisdiction to jurisdiction. It has to be noted that non payment of the renewal fees will lead to revocation of the patent.
Further, the fee payment status is being shown as paid because renewal fees of the patent has been paid till the patent was in force.