[Lingo] 上手･上手い

[steve smith <sjs@example.com>]

I recently received an email from a friend that used the word 上手い・う 
まい.  It's the first time I'd seen it but was familiar already with 上 
手・じょうず.  These two look pretty similar in meaning with the 
exception that one is an i adjective and the other is a na adjective. 
Does anyone know if there is a reason for having the two words other 
than 'it just happened that way?'  I'm sure there are are other words 
like that and I'm just curious.

Thanks
Steve S.