Similar to english colloquial use of "on top of it" e.g."Not only is the movie theatre cramped, on top of that (previous condition), the screen is also small."
May I suggest maybe another explanation for the grammar point on this page?
"On top of A, B" or "In addition to A, B"
The current explaination on this page is "Not only A, but B as well" which kind of gives me te idea I could do a phrase with it like "Not only Sara lies, but Carl as well". Now this sentence probably cannot be made with 上に and maybe me interpreting the current explanation that way is a resoult of my poor english, I don't know, fact is that I think the two I suggested are easier to understand (for me, and hopefully for others as well :S)
