guys i just figured it out for the fire!!!!!!!!!
how to remove the unwanted outside downloaded residual cache in your internal memory!
just follow these steps:
1. go to gearwheel on top right corner
2. choose +more
3. select applications
4. on filter by bar, choose 'all applications'
5. then scroll down and choose media storage
6. choose what's need to erase
in my case i completely erased all movies and songs, and my internal memory still read the same even with folders empty. i then went the route above and clear data for storage. to my surprise, all my memory came back, looks like the 'residual' stuff from outside downloaded movies was also erased. this however, i have not applied with selected movies. still advice just as others did in forum...to save content on cloud and use the above steps if your going to use the fire for shuttling wanted movies back and forth only.
please ad on this statement and do more research on this to help out others.