Actually, if you have a legacy Delta system, you are REQUIRED to have it grounded somewhere now, or have a Ground Fault Monitoring system. The most common grounding method, if done, is to corner ground it. My point was that if that is a two transformer delta service with a corner ground, and NONE of the three phases is reading at zero volts referenced to ground, then it is floating. And if one of them is floating at 763V referenced to ground, that is (likely) a "ghost" reading in that it is just the capacitive coupling to the ground reference, but if you see that, then that means something ELSE is seriously wrong.
But as to it being "High Leg" delta, no way with the data provided. If it was 480 Line to Line (as stated) and one phase was center tapped to provide 240V Line to Neutral (weird), the "High" leg referenced to ground then would be the sq. rt. of (480^2 - 240^2), so 416V and the other two legs would read 240V referenced to ground, not 400V. There is no math formula that gets the 400V and 763V values. Something is broken somewhere.