That could be the reason. You could confirm this by examining the properties of this controller. First, press the Start button in Windows, and type "game controller" ... select the "Set up USB game controllers" utility. Next, select the game controller you want to view in this list and press the 'Properties' button:
From the Properties screen, under the 'Test' tab along the top, we can see every button and axis of this controller. Move the control you are curious about and observe what it affects in the Properties screen. If it is indeed an axis, it should be apparent here - an axis cannot be set as a Joystick button press trigger for a command in VoiceAttack: