Windows creates all sorts of temporry files, some of them quite large.
The System Restore feature in Windows can create a large file, and can use up to 15% of your disk space.
Maybe there were some old System Restore files that Windows deleted for example.
Windows also has a large hidden file called the pagefile that shrinks and grows as Windows needs it, maybe that was smaller than usual.
Remember there are all sorts of things going on behind the scenes with Windows that most of us dont know about so just because YOU did not delete some files does not mean it did not take place.