Maybe there are some hidden files - are you selecting all the files 'manually' or with command-A? Does that make sense Mac gurus?
Unlike Windows, Mac doesn't have the concept of hidden files that you have to turn on viewing for in some obscure dialogue box.
(Well, strictly speaking, being a BSD derivative it does, but would require object names prepended with a dot ..... which in the circumstances under discussion I would think to be fairly unlikely. However you can easily check by firing up "Terminal" cd'ing to the SD card volume and playing around with "ls -la").