I think you should be 100% sure that your measurement method is correct before contacting the manufacturer. In your place, I would set the generator to 100mV RMS, 400Hz, confirm it by measuring with DMM, additionally confirm the perfect sine wave shape on the primary with an oscilloscope, set the input pot of the scope to cal !!! (how do you know what voltage you are measuring if the pot is not at the cal position?), then measure the voltage at the secondary, it should be twice as high, and confirm it with an oscilloscope.