Cards' width (it's the same for all cards in a page) is between min_card_width and max_card_width that are two parameters an app (in this case Discover) developer defines and Kirigami uses them to calculate columns_number, card_width and additional_margins (he ones on left and right between cards grid and page):
1. columns_number = page_width / (min_card_width + margins)
2. card_width = ( page_width / columns_number ) - margins
3. additional_margins = card_width - max_card_width
to reduce additional_margins the app developer must reduce the difference between min_card_width and max_card_width
in this image you can see how max_card_width is increased to fill the space taken by additional_margins but it doesn't look very good. So what's the solution? Changing card content layout as you see in the last card!