When a tree with a node with sub-nodes is created, in the source you can see a end line between the two images. This end line create a litle space between the two pictuce as you can see in the capture 1 of the showcase. For example in the tree of the iceFaces showcase at the first generation of the code :
<div ...> <img src="tree_line_blank.gif"/> <a ...><img src="tree_nav_middle_close.gif" alt=""/></a>
<div ...><img src="tree_folder_open.gif" .../></div>
<div ...><a ...><span ...>Western</span></a></div>

But now if you collapse and expand this same node, the code became :
<div ...> <img src="tree_line_blank.gif"/> <a ...><img src="tree_nav_middle_close.gif" alt=""/></a><div ...><img src="tree_folder_open.gif" .../></div><div ...><a ...><span ...>Western</span></a></div>

With the absence of end line, the space disappear and the icon in the tree move a little as you can see in the capture 2 of the showcase.