In BLDC sine commutation, why are the low and high of each phase inversions of each other?
Look at one of its (other 2 seem similar). You don't need to think too complicate let's think this way. it's 4 N-chanel FET.


If read signal go high, high side of phase will conduct, and phase voltage be +V.
If green signal go high, Low side of phase will conduct, and phase voltage be -V.
The motor act like Low-pass filter so it average signal voltage so we can calculate from duty cycle of V+ and V-
To make this assumption true, we must keep phase voltage V+ Or V- so we can average its.
If you make float state (high impedance) the formula  can't be true because we can't define voltage of floating pin (actually it will depend on Back EMF) to make it precise sine wave it has to done this way.