Shutter actuations count including when there's no memory card in, and also when doing a sensor clean. And if you go over 10k shots then the image count number will have rest so you get a very inaccurate figure using that information.
The EOS Info app will give you the shutter count for a 50D, it's the successor to the 40D shutter count app mentioned above:
http://astrojargon.net/EOSInfo.aspx