The fare matrix shows the lowest fare even if it’s at an alternate airport. Hover your mouse over the fare in the matrix and Deem shows more details:

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To see the alternate flight options (other airports), click the fare in the matrix, or click ‘View all times and airports’ in the search box. Or (to leave the times the same), select individual airports:

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Then it shows alternate airports, indicated with an exclamation mark:

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