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Sharpness and Dxomark score differ slightly when considering sharpness in the field. The sharpness score tolerates a loss of sharpness in the corner when DxOMark score does not.
If you take a look to the sharpness in the field for these two lenses, you will see that the Sigma lens loses some sharpness in corner hence the DxOMark score comparison.
The DxOMark team