FCP X does not transcode any media that it considers optimized for FCPX. It sounds as if 40 of your 50 shots are already optimized for X, leaving only 10 that needed to be transcoded. You can check this by opening one of the non-optimized clips (ie: one that is not duplicated in the Transcoded Media folder) in Quicktime. Use Get Info to read the codec information. Do the same with one of the clips that IS duplicated in the Transcoded Media folder. Check for a difference in codec, or codec/frame-rate.
Andy
http://www.timesavertutorials.com