From my site.
Paging feature explanation:
If the tree grid has no selection then paging toolbar shows information related to grid's root nodes set, e.g. if it shows "Displaying 1 - 10 of 20" then there are 20 root nodes (nodes with root id === null) which are divided into two pages. If the tree grid has a selection then the paging toolbar shows information related to selected node's child node set, node must be expanded for node's paging related information is loaded. After node expanding paging toolbar shows selected node's child node set related information and the paging facility are applyed to that node's child node set only. To start paging at the root nodes you must deselect all nodes first.