The SSD optimizer works by letting the SSD know which Logical Block Addresses (LBAs) are associated with already deleted files. It should be "blind" to your OS -- you should not see a change in the amount of free space reported on a disc.
The fact that Windows is reporting that 21GB resides within the folder eliminates the SSD optimizer tool as the problem. I suspect that you may have some hidden files or something that is taking up the space yet the folder "seems" empty.
Can you kindly attempt the following:
Start --> Run --> Cmd (to open up a DOS prompt)
Navigate to the directory of interest.
>> dir /a (directory command with the "show all" switch)
I suspect that you will see files here.
I already tried this, the folder is extremly empty, yet the OS reported 21 GB of data in 2 folders and 44 files. There are no folders and no files here, no hidden folders and no system folders.
I find this extremly wierd and is beginning to wonder if there is some kind of bug in the win7 driver... because when I reboot the PC, the correct amount of disk space is reported. Or the SSD is flawed somehow.
This might have nothing to do with SSD drive. I've seen similar issues before on HDDs (e.g. http://www.bleepingcomputer.com/forums/topic130053.html )
Is it possible to replace ssd with hdd just for testing? i think it's just an OS bug