I volt that electrofelon and gar have it.
There is no such thing as an ungrounded system. What we call ungrounded, if there are no faults, is really _capacitively_ grounded through all of the unintentional capacitors formed between the circuit conductors and ground.
A wire in a conduit is a capacitor. The magnet wires in a motor and the steel of the motor? Another capacitor.
The (very, very small) leakage through the insulation and the air around the conductors? Resistors.
An 'ungrounded' system in perfect condition will have very small _wye_ currents to ground, through the above 'parasitic' components. If the system is in perfect condition, and you measure with a high impedance meter, then I'd expect voltages quite similar to those from a wye system.
If you measure with a low impedance meter, or connect a load from phase to ground and measure voltage across that load, then the voltage to ground will be much lower than 277V.
If there are faults or intentional wye resistances (for example old school ground fault detection would be with a set of lamps connected wye to ground) then you might need a very low impedance meter to get a low voltage measurement.