Voltage, by definition, is the potential difference in charge between two points.
In your image, Point R, by itself, does not have a voltage because it is only one point. Point R with respect to Y (RY = two points) does. Point R with respect to B (RB = two points) does as well. Point R itself can be apart of more than one voltage simply because you require two points to determine a voltage. In a delta system, the two points required to measure a line voltage also happen to be the same two points connected across the phase.
I think your confusion may come from being familiar with wye systems. Don't let the architecture fool you though! In the image below, Point A has no voltage on its own. The phase voltage is still from Point A to Neutral (AN). In the case of the wye, the line voltage is between points A and B (AB).
