The fourth edition of Catan and its related expansion was originally published with tiles of a particular thickness. When they decided to publish the Explorers & Pirates Expansion, they felt they needed thicker tiles due to the constant tile flipping. So they started printing the base game and all the expansions (with the exception of C&K) with thicker tiles to match in 2012. -- My source is a phone call with the publisher. I have two copies, one of each thickness.
While you can use the thinner tiles from Catan (and expansions) with E&P, it is better to have the version of Catan (and expansions) that have the thicker tiles.