The RMS voltage is based on potential difference between the two poles. The actual potential difference varies between 0 when both waves are crossing zero to 340 when both are at their peaks. The RMS is 240 (nominal).
Don't confuse measuring potential between two coincident peaks of separate waveforms with measuring peak-to-peak (necessarily non-coincident) voltage within a single waveform which is something quite different.
For a single pole to neutral, the potential varies from 0 (zero crossing) to 170 (at peak).
