:nth-child(5n) is not working properly

:nth-child(5n) is not working properly

Can anyone take a look at http://www.vitaminwatercanada.ca/gallery/most-popular.html and tell me why the 4th elements are getting selected by 
$(".entry-summary:nth-child(5n)").addClass("last"); 


You can try the following code in Firebug console to debug: 

$('.entry-summary').removeClass('last');
$('.entry-summary:nth-child(5n)').addClass('last');