It is likely that your substation transformer has a "floating" delta secondary. By this I mean that the three secondary transformer phase windings are all wound for 460V and they are connected in a delta configuration. Since there is no neutral point, you have a 460V three-phase system but no reference to ground.
If the leakages and capacitances to ground are the same on all three phase wires, then the system will normally balance itself at around 277V to ground on all three phase wires. However, if there is a difference between the resistance or capacitance to ground (the most extreme case would be one leg solidly grounded) then the voltages from phase to ground on the remaining two legs would rise even as the leg that is being pulled to ground has less voltage. Remember that the substation secondary windings enforce the 460V from phase to phase but could care less about where ground is.
The other common substation transformer secondary arrangement would be for the transformer to have three secondary windings like before but they are each wound for 277V. One end of each winding is attached to ground and the remaining free ends become the three phase output connections. This configuration also has 460V phase to phase but the 277V balance to ground is rigidly enforced by the transformer windings. The difference here is that, if one phase is pulled toward ground the current rises and soon an overcurrent device will open. On the floating delta system, the condition doesn't open anything and the service continues to operate even tho it is out of balance.
Depending on who you talk to, the floating delta system is considered good because production is not interrupted for a single ground fault OR the floating delta system is considered bad because it overstresses line-to-ground insulation and can be a hazard to personnel. For myself, having almost been killed by a floating delta system running with one leg grounded, I'm not fond of it at all.
Today, there is increasing use of grounded wye systems using a resistance tie to ground to limit fault current, low resistance to eliminate catastrophic failure from high currents on ground fault, and high resistance to permit continued operation similar to the floating system on a single ground fault.
In any case, with any system that can tolerate a phase lead ground fault, it is essential, in my opinion, to have proper alarm equipment on the substation so, when imbalance occurs, even tho you keep running, an alarm is produced and repair procedures are initiated as soon as the power system can be shut down for testing. Running a system deliberately or ignorantly (with a very few exceptions like hospitals, etc.) with one leg grounded is just plain negligent.
When I start up equipment, if I find the power supply more than 25% out of balance, I will refuse to complete the startup until it is brought back in balance or an isolation transformer is found for my equipment. I've made a few enemies that way but at least I'm here to talk about it!
