Weird disk performance stats

I'm using Defraggler 2.18.945 (64-bit). I noticed that a recent update has added the Statistics tab, so I used it to measure the performance of all my drives. And they are not at all what I'd expect.

My PC has three SATA/300 drives, C, S and T. There is also a USB2 hard drive plugged in - drive J.

I'd expect C, S and T to have fairly similar speeds, and J to be somewhat slower.

But the graph of the speeds (see attached) shows the C drive to be very slow and the speeds of S and T to be much faster. On several different runs, S is sometimes faster than T and sometimes slower: the example shown here has S=116 MB/sec and T=84 MB/sec. C, the system drive, is 1.3 MB/sec - VERY slow.

When I've copied large (eg 1GB) files between two folders on the same drive, I've found that all three drives are about 30-50 MB/sec. J, the USB drive, is about 20 MB/sec.

Sadly the help files at http://www.piriform.com/docs/defraggler don't mention the various tabs such as Health and Statistics - I wonder if they need to be updated to keep pace with the product :-)

All four drives are NTFS, by the way.

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I am seeing similarly strange numbers, with my C drive being by far the slowest, and one of my partitions on the same drive being shown as about 25x as fast. Likewise, my external USB 2 drive is shown as being twice as fast as the system drive... Something is definitely wonky here. We could use an explanation for these numbers. Does this maybe have to do with the cluster size the drives were formatted with?