Well actually there are three rating fields a plugin/script can use: rating, imdbrating, orating
The last one is special because it is a custom rating field for the script.
I guess the TMDb plugin for some reason uses the imdbrating field!?
My version of the script (
http://www.videodb.info/forum_en/index.php/topic,1030.msg15869.html#msg15869) uses the orating field and sets the rating name to TMDb.
I dont know if this has any implications but from my POV this is the right way and is actually a bug in the plugin.