3 Phase Delta with Ground Reference?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. "Isolated" delta supply with ground-fault monitoring.

Without a wiring diagram of your supply it is impossible to say. One possibility is that there is a neutral connection and an unbalanced load. Figure 1 shows a common setup where three incandescent lamps are used to monitor the three phases:


If there are no earth faults on the system all three lamps will illuminate at the same brightness.
If an earth fault occurs on one phase the phase lamp will go out and the others will be brighter.


The lamps, in effect, create a weak pull to centre around the neutral. (Don't forget that neutral is connected to ground at the incoming supply point or at the local supply transformer.)

If you do a CAD drawing of an equilateral triangle with sides of 120 V and strike arcs from each vertex - one of 77 V and two of 65 V they should intersect pretty closely. That is your "neutral" point which has been pulled away from the true centre by an unbalanced load.