hard drive - Windows 7 Disk Cleanup deleted 30GB of temp files but only recovered 10GB of disk space?

Windows 7 was reporting I had 9.49 GB space free on my primary C: (SSD) drive:

Windows Disk Cleanup reported I had 28.1 GB of "Temporary files" that could be deleted to get this space back (together with 206 MB of "Temporary Internet Files" and 154 MB of "Thumbnails" gives the 28.5 GB "Total" in the screenshot below).

I proceeded with the "Disk Cleanup" (logged in as an Administrator user). However, even after rebooting, the recovered space is only about 10 GB:

What's happened to the other 18+ GB? Checking "Disk Cleanup" again, now shows "Temporary files - 0 bytes" - so the files do appear to have been deleted - but why haven't I gained the amount of disk space I was promised?

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UPDATE: After some digging, a number of other questions/sources seem to suggest that the amount of freeable space by Disk Cleanup is, at times, grossly overestimated?! For example:

Although, I did not first (manually) check the reported disk usage for the various locations Disk Cleanup uses when calculating the amount of "Temporary files", so I cannot confirm this.