This is the normal behavior with 2D plotting functions, the values are rescaled to lie between 0 and 1 before being fed to the color function. If you use the option PlotLegends -> Automatic you can see the color mapping that is being used. Use the option ColorFunctionScaling -> False for your ArrayPlot call to turn off the automatic rescaling.

See http://reference.wolfram.com/language/ref/ColorFunctionScaling.html, or https://mathematica.stackexchange.com/search?q=colorfunctionscaling for more information.

If automatic rescaling is turned off, then the coloring is exactly as given by your ColorFunction. If you say ColorFunction -> myColorFunc, ColorFunctionScaling -> False, then 0.3 should be colored exactly as myColorFunc[ 0.3]. Use PlotLegends -> Automatic to see a color bar.