This is another box
This box has more content than the others. If all boxes were table cells, the cell with the most content would decide the height of all cells. It works like that here too, but this is not a table.
Instead, display:table, display:table-row and display:table-cell are used to make divs behave like table cells. Excellent. Too bad it doesn’t work in you-know-which-browser. It does, however, work in modern browsers like Mozilla, Opera, Safari, Firefox, OmniWeb, Camino, and Netscape.
Read the related blog entry for more info: Equal height boxes with CSS.