You are looking at the same bank and same page on both sides. The page selectors are at the top and look like tabs, and the bank selectors are the little numeric things that say "1" (or "2") at the top of either side. See the pic below.
Pages exist in banks, and you are free to show the same bank on both sides and even the same page (useful sometimes in Design Mode).
To stop this from happening, switch the page on the right side (or left side). Alternately, you can switch the banks to be separate on the right and left sides. If you do this, you would have two different numbers showing at the top (1 and 2, for instance), and each side could access totally different pages.
Note: you can add and remove pages and customize them (color, texture and name). You can also add and remove banks.