If the rows do not all contain an equal number of elements, when in a column with fewer elements the up/down wrapping will place you on the last element of the last row.
Steps to reproduce:
1) Open the lightbox demo from the daily build site:
2) Adjust the browser window size so that the images are laid out with the bottom row having fewer images than the other rows
3) Using the keyboard navigate to one of the shorter columns of images. Press the down arrow key. Notice that the at the end of the column, pressing down will place you on the last image of the last row instead of wrapping you around to the top.