This is a known issue with digital cameras or camcorders when you use the computer to delete the video or still image...
When the camera captures a still image or video, there is an internal "database" that is updated. The information includes things about the data and time of the capture, size of the video and a bunch more stuff. It helps the camera play things back, use camera transitions and other capabilities. If the camera is used to delete the still image or video then the database gets updated. If a computer is used to delete the still image or video, the database is not updated.
If the database is not updated, then the camera thinks the memory is in use (by the deleted file)... The resolution to get all the memory back is to use the camera's Format command. In the future, use the camera's delete function - or if you continue to use the computer to delete the files, just remember that at some point, to recover the memory space, you'll need to copy all the video and still images to the computer and format the camera's memory with the camera's format command.
Once the camera's format command is used, a new folder structure will be set up along with a new database. You *cannot* move still images from the computer to the camera as the camera's database will not be updated and will not recognize the files. This is why you want to copy *everything* off the camera and onto a computer BEFORE you format the camera using the camera's format command.