Four number-pairs have been given, out of which three are alike in some manner and one is different. Select the number-pair that is different.

797 : 679 373 : 255 727 : 609 191 : 177

191 : 177

The logic followed here is :  ist number - 118 = IInd number 797 : 679 ⇒ 797 -118 = 679 373 : 255 ⇒ 373 -118 = 255 727 : 609 ⇒ 727 -118 = 609 191 : 177 ⇒ 191 -118 = 73 ≠ 177 (odd)