Multiframe bitmaps "normalized" value doesn't make sense?

[Nonlinear]

How come when I have a bitmap with 3 frames I need to send value "0.7" in GetGUI()->SetControlFromPlug(int controlIdx, double normalizedValue) to get the 2nd frame to show? If the value is "normalized" why doesn't it respond like this:

frame 1: 0.0 - 0.33
frame 2: 0.34 - 0.66
frame 3: 0.67 - 1.0

If I send value 0.5, for example, I get frame 1! Can someone explain this?

Alternatively, is there any other "SetControl" function for INT rather than normalized float for bitmap controls?