Lenses are labelled based upon performance at infinity focus. Many very complex lenses (e.g. superzooms) lose focal length as you focus closer, a trait called "focal length breathing." Optical designs balance an enormous number of variables, and in aggressive designs (e.g. superzooms) something has to give. What "gave" in the 18-200mm design is focal length reach at close distances. The very first superzoom—the Tamron 28-200mm back in film days—also made the same compromise. So the 18-200mm's performance isn't anything new. Likewise, the 28-300mm FX Nikkor has a similar problem.
Note that the 70-200mm f/2.8G AF-S VRII also has a form of focal length breathing, and this is obvious when compared directly to the original version of the 70-200mm or the most recent 70-200mm f/2.8E FL. See my review of the VRII version for more details.