Looks like you missed one in March:

http://www.uscoscots.org/Festival_News.htm

Although from the City of Tulsa Pipes and Drums page it says:
The CTPD is a fixture at the Oklahoma Scottish Games every third weekend in September at River West Festival Park where many individual members work hard to bring a competitive piping and drumming event to the Tulsa area.
So perhaps the games are the third weekend in September and they are just a wee bit slow in updating the tulsascottishgames.org website? They've had them for 28 years so I don't think they would just up and quit doing it.