has Windows turned off System Restore during the running of Eraser? There are instances where this has happened as the amount of free space runs low during the Eraser running process and this can free up a fair bit of space.
For instance, the last time I turned off System Restore (I always do prior to an Eraser Free Space run) I gained nearly 400MB of free space as I had installed a few programs and S.R. can create a Restore point every time you take a certain action (and it is programmed to create a Restore point at set intervals anyway).