If voltage is measured relative to something. For instance, voltage-meters measure voltage differences not individual voltage levels, or by analogy altitude is measured relative to somewhere. Mount Everest is so-and-so compared to sea level. But zero compared to itself.
How come line to line voltage is not zero when both lines have the same value? For instance, a three phase three wire Y connection of 120V with an angle of 120 degrees. Line to line voltage here is $$120V \times \sqrt[]{3} \approx 208V$$ not zero.
Best Answer
The difference between +1 and -1 is 2 yet both have the same magnitude of 1. If you connect two identical 9 volt batteries together with a single wire and measure across the unconnected terminals you might measure 0 volts or you might measure 18 volts depending on how you connected the single wire. Polarity matters and it matters in 3 phase systems just the same.
As you can see, although the 3 individual phase voltages are rising and falling identically, they are displaced in time and therefore there is a voltage between any two.
Picture stolen from here