I am not completley sure what is the exact problem biut I assuem that it comes from the fact that you have a RadTabStrip in one of the RadPanes and at the same time you have a RadMultiPage which is set to 100% Height. This means that the MultiPage will occupy exactly 100% of the pane and because the tabstrip has also some height, it will push the multipage down with this height.
Note, also the following things:
1) Do not set any sizes to the nested splitters if they are directly nested in a RadPane - when a RadSplitter is directly nested in a RadPane, it automatically resizes itself to occupy 100% of the parent pane. If you set width, height, fullscreenmode, the splitters will make a lot of calculations to resize themselves and this is not necessary and slows down the performance significantly.
2) If the nested splitters are not directly nested in a RadPane but in another element (in your case - page view), set their sizes and also set the ResizeWithParentPane property to false.
3) If you have a Radpane in a vertical splitter do not set its height explicitly to 100% (respectively the width for horizontal splitter) due to similar reasons as explained in 1)
I hope that my explanations and suggestions are detailed enough and helpful and for your convenience I modified and attached your demo page to the thread - let me know how it goes.
All the best,
the Telerik team