It looks like you formatted the column with thousand separator and decimal point. Thus, the Buttons extension exports those values as text. Nevertheless, you can try this export option to see if it removes the comma and period symbols to export those values as numbers:

DataTables::create(array(
    ...
    "options" => array(
        ...
        "buttons" => array(
            array(
                "extend" => "excelHtml5",
                "exportOptions" => array(
                    "format" => array(
                        "body" => "function ( data, row, column, node ) {
                            if (column === 2)  // change 2 to the order of your numeric column
                                data = data.replace( /[,\.]/g, '' ); // remove all commas and periods
                            if (column === 4)  // change 4 to the order of your numeric column
                                data = data.replace( /[,\.]/g, '' ); // remove all commas and periods
                            return data;
                        }"
                    )
                )
            )
        )
    )
));