(Short, incomplete answer; this question deserves a better one) 

*Theism* and *Deism* are fairly "jargon"-y words. Jargon doesn't have to have a logical etymology; the point is to have any kind of label at all. E.g. the prefixes "a" and "im-" usually mean the same thing, but people have come up with a special distinction between "amoral" and "immoral".

So I would say there is no etymological explanation. The Oxford English Dictionary entry for "deism" cites a source that says that:

> 1877   E. R. Conder *Basis of Faith* i. 25   Deism should etymologically have the same sense with *Theism,* but it is commonly taken to carry with it the denial of what is called revealed religion. Theism conveys no such implication.


  [1]: https://en.wikipedia.org/wiki/Ignosticism
  [2]: http://www.thethinkingatheist.com/forum/Thread-Igtheism