10 has two factor pairs:
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560 is one such.
The difference is between factor pairs and distinct factors. With square numbers, one of the factor pairs will be the same number twice. When listing the distinct factors, that number is only listed once.
(y + 3)(y - 10)
The factor pairs of 20 are: 20,1 / 10,2 / 5,4
The factor pairs of 51 are (1,51) and (17,3)