1. Competition is tougher (more serious players in TWC, less casual ones).
2. Quicksearch is based on PR level from the very start, so you don't get n00b bash (as much) for months on end.
3. The combat is more brutal (alot more powerful units running around).
4. The game is less balanced, so if you fight with a disadvantaged civ against the top ones most of the time, your rating can be run into the ground quickly.
5. Lastly, ratings inflate slowly, and it's only been a few months since TWC release. They will eventually get close to what they were in vanilla.
Now forget about ratings and play for fun, experiment, go nuts, and play with the civ you enjoy. Nobody, not even you will care about what your rating was five years from now.
"One wants to be loved, failing that admired, failing that feared, failing that hated and despised. One wants to instill in other people some form of emotion. The soul shudders before emptiness and wants contact, no matter the cost."