Neutral Point Diode Clamped Inverter Switching States
You are being confused by the rather cryptic drawing and logic table. 

Note the output is termed \$V_{aN}\$ That implies the measured output is the voltage difference between node \$a\$ and node \$N\$. Since node \$N\$ is already biased at \$V_{dc}/2\$ when the output is high \$V_{aN} = V_{dc}/2\$, when the output swings to the other rail  \$V_{aN} = -V_{dc}/2\$.  

However the truth table seems a tad messed up. From my understanding output low should be 0,0 on the inputs. Further the state 1,0 should produce open circuit, or high-impedance, output.