Why QPSK31 uses the same 31.25 baud symbol rate and shaping as BPSK31, so it occupies roughly the same narrow bandwidth of about 31 Hz. The difference is that it uses four phase states instead of two, and the extra bits carry a convolutional code that a Viterbi decoder uses to correct errors. Because the receiver must track the direction of phase rotation, QPSK31 is sideband sensitive: pick the wrong sideband and the decode fails, unlike BPSK31 which works either way.
Watch out Each individual statement is true, so choosing only one of them, such as the bandwidth statement, leaves out the error correction and sideband sensitivity that also define the mode.
QPSK31 = BPSK31 plus two extras: error correction and sideband sensitivity, same 31 Hz width.