You'll probably find the most accurate explanation at the CK mining pool page.

https://bitcointalk.org/index.php?topic=5237323.4660

From page 234. Personally I love to see things like this.  Sure the chances of finding a block are tiny but it's a great way to learn and maybe get into larger scale mining, these small devices don't use much electricity, there's a cool solo mining community on the thread I linked above, and solo mining helps the network stay decentralised.  I think there's too much emphasis these days purely on what is profitable and what isn't, takes a lot of the fun out of it.