Unbalance Voltage

A mismatch in the impedances in each of the phase loads would do it.

E.E.65 is right, voltage balance of secondary on such a transformer is not something to bother about. In practice, the LV windings have insulation capacitance to ground AND to the HV windings. So by capacitive voltage division, the HV voltage can throw unexpected voltage into an un-grounded delta secondary.

Real transformers have variations between phases (even by design) and unbalanced input voltage and your voltmeter will respond to harmonic frequencies at which input unbalance may be worse than for fundamental - so balanced voltage to ground is unlikely, rather balance would be exceptional.

The only reason to worry is if one phase voltage to ground is very low - which could be an actual earth fault.

You have not written that your transfo is loaded - loads are often single phase and capacitance and leakage to ground of the phase loads are often unequal because the loads themselves are unbalanced.