Why is disk fragmentation measured as a percentage?

In Windows if I open up the Disk Defragmenter and analyse a disk the tool reports the disk fragmentation as a percentage:

What does this percentage mean? Obviously if a drive is 0% fragmented then there is no fragmentation at all, however what does it mean for a fisk to be 100% fragemented? Is this even possible?

Fragmentation is where a file is not stored within contiguous space on the hard drive, and therefore split into separate parts (fragmentation).

This typically occurs because files have been deleted, and leave a specific size space, however when the next file is saved it will not completely fit into this allocation and is fragmented over multiple areas on the hard drive, because the file is too big for the space.

The percentage I assume is saying that 2% of the files on your hard disk currently have one or more splits, and are therefore not stored contiguously.

I doubt that 100% fragmentation could occur, as there are some files at the core of your OS that will probably never change and be installed from initial installation. Also there are many small files on your operating system that should never require fragmentation.

It could however be possible for 100% fragmentation on a second hard drive that only holds large files, as there is no operating system here.