SDL_SetGamma only half working

using SDL_SetGamma( … ) returns 0 (which means it works )
but the thing is it only actually changes the gamma if the numbers are in the range 0.0->1.0
eg SDL_SetGamma( 0.5,1.0,1.0 ); makes red half bright
but SDL_SetGamma( 2.0,1.0,1.0 ); is the same as SDL_SetGamma(1,1,1) ie the 2 is clamped to 1
is this to do with my hardware setup?

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