It's possible because in order to wipe the free space of a hard disk drive the entire free space must be written over multiple times with random data.
Therefore the amount of data shown by the Disk Wiper can be larger than the actual amount of free space, and also this usually is the case.
The process is very heavy and with a large disk with many gigabytes of free space it will take many hours to perform the task to. It's usually a good idea to leave the process to run over night.
