Yeah, WaW was a Treyarch release, not IW.  They alternate releases. 

CoD - IW

CoD United Offensive Expansion - Treyarch

CoD2 - IW. 

CoD3 Big Red One (console only) - Treyarch. 

CoD: MW - IW (Which was graphically a great game, but I hated the way all guns went to both sides.  I liked the balance of the old WWII weapons being country specific.  Better balance IMO)  It was at this point that I dropped the game altogether.

CoD: WaW - Treyarch (played it SP, wasn't impressed)

CoD: MW2 - IW

(I think at this point, they realized the Treyarch games were seriously inferior)

CoD: Black Ops - IW

and so on....