Why is the combined intensity of two wavelengths greater than the mathematical sum?

Synergy is a partial answer, but emergence is more accurate. Emergence refers to the way a complex system or systems and patterns arise out of a multiplicity of relatively simple interactions. Some (myself included) would argue that this is also an off-shoot of Chaos Theory (also known as The Butterfly Effect or Sensitive Dependence on Initial Conditions) My somewhat play-full theory is that our inability to measure these factors to a six-decimal (or more) base allows for too much of a variable in the outcome of the equation. As a friend of mine like to say: "It's not reality...it's physics."

Are you sure of this?