This could mean that stuff came down from the cloud to your hard drive as well as going up into the cloud.
I received a warning that I was running out of disc space. I "moved" not "copied", 64 gigs of photos and videos to an external HD. My Laptop HD then showed that I had 66 gigs of free hard drive space. I then ran Amazon Drive overnight and chose to sync. Today I am back to 2 gigs of HD space. What might have happened? Everything I moved is on the external HD and no longer on my Laptop