RGB LEDs

So, on an RGB LED fixture I can program using the CMY encoders and have the ability to use the Intensity encoder too, or I can program using the Red, Green, and Blue encoders under the Intensity parameters, but can not use the Intensity wheel with this.
Here's my question....I noticed that the LEDs on a SHOWGUN can be programed using Red, Green, and Blue encoders along with a Master LED Intenisty encoder. Why can't an RGB LED fixture have a Master Intensity parameter that will work when programming using RGB and not CMY?
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