Bi-mettalic overloads trip based on current (heating) only. Voltage has nothing to do with trip settings.
I believe you are misunderstanding the meaning of the wye/delta scale. There appears to be a ratio of 1:1.77 between the left and right sides of the scale, but this is not the same as 1:2 voltage ratio you inferred. My feeling is this scale has to do with application on a wye-delta reduced voltage starter and not between normal high and low voltage motor winding connections.
