Three of the following four number-pairs are alike in a certain way and one is different. Find the odd one out.

21 : 484 25 : 676 24 : 529 30 : 961

24 : 529

21 : 484 = 21 : 222 25 : 676 = 21 : 262 24 : 529 = 24 : 232 (ODD) 30 : 961 = 30 : 312