If the variac is simply a variac (i.e., an autoformer intended for manual adjustment of AC voltage), for any given setting the output voltage will vary in the same proportion (i.e., by the same percentage) as the input voltage.
Some variacs, however, are contained in instruments which also include mechanisms and circuitry that automatically adjusts their setting such that a stable output voltage is maintained. Although those adjustments, when required, will not be instantaneous, and may be accompanied by a significant amount of mechanical noise.
Regards,
-- Al
