The second number in the given number-pairs is obtained by performing certain mathematical operation(s) on the first number. The same operation(s) are followed in all the number-pairs EXCEPT one. Find that odd number-pair.

100 : 9

The logic followed here is :

25 : 3

⇒ 3 + 2 = 5

⇒ 5² = 25

64 : 6

⇒ 6 + 2 = 8

⇒ 8² = 64

100 : 9

⇒ 9 + 2 = 11

⇒ 11² = 121 ≠ 100

81 : 7

⇒ 7 + 2 = 9

⇒ 9² = 81