Why is the voltage more in case of same polarity during a polarity test of induction motor?
If the windings in the motor were windings of a transformer, you are probably right - but this is a motor and therefore the two windings in serie are wound in opposite directions and may not be considered as additive. 
The windings don't share the same iron core position either so the magnetic fields from the different field windings are not 'in line' as they are in a transformer.
It seems like you are messuring the input voltage + the induced voltage of opposite polarity from the second winding.
Looking forward to any comments