My guess is at high throttle the injectors could not keep up with demand, as they would have to pass- what was it, about 25% more fuel?- to match the BTU requirements of regular fuel.
Actually it's the air/Fuel ratio that has to be lower not the BTU content that is the problem so you may have been running too lean and having fuel ignition problems. Certain fuels can burn over a wide air to fuel ratio and some can not.
Nope it is because of the chemical makeup of the fuel and the amount of oxygen molecules that are needed for it to burn properly - Butane requires a lot more air but has a lower btu content. Methanol I think needs like 8 to 1 ratio or something low like that.