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The construction process makes the difference, i.e. the second ko-shape loses a shared liberty, compared to the larger eye.

The moves from to , displayed in the diagram, are the relevant ones for building the basic shape of the second ko in the centre.

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In this diagram, the moves from to are relevant for building the basic shape of Black's larger eye.

You will realize that here is one shared liberty more () than with the second ko-shape above.

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A move pair (/) that reduces Black's group with the larger eye to eight effective liberties (resulting in the need of + = 4 ko-threats for Black, to win) ...

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(Referenced by 1159)

... will reduce Black's group with the second ko-shape in the centre to seven effective liberties (resulting in the need of + = 3 ko-threats for Black, to win).

Here, Black needs one ko-threat less than with the larger eye, making up for the loss of the internal ko-threats, he has there. So, with both basic shapes, Black needs three "external" (i.e. outside his eye) ko-threats, but has only two available in the problem.

Both basic shapes are equivalent!