How does R-Studio recover thousands of deleted/"lost" files from a SSD with no free space

I have a Samsung 850 EVO for a Windows 7 64-bit boot drive, with a single partition. There were 14,478 JPEGS (as well as thousands of other files) in approximately 180GB of free space on the SSD, totaling just over 1GB. I copied PDF files to the SSD until there was no room left on the drive. I had assumed that the files would be overwritten. However, ruing R-Studio again found the JPEGS, and it was possible to view the majority of the them in the viewer.How is it that these files persist?