Hi VB,
Thats it.
It requires heat/energy to vaporise the fuel,so as to burn it.Only the vaporised fuel is burned.
It goes something like this;
-Compression of fuel/air mix heats it up.
-Ignition occurs burning existing vapors which vaporises more of the fuel which then burns etc until the charge of fuel or oxygen is gone.
It's like burning the fuel off a layer at a time until all the layers are gone.There is a name for this that I can't remember, will check.
If you can imagine a miniature pot of fuel(representing the fuel from a single intake stroke) sitting on a mini bunsen burner you will get the idea of the heat energy used to vaporise (boil off) the fuel charge.
So with Nitro the boiling pt is higher and there is more of it (remember it uses less air but the air quantity is fixed in ic by stroke) thus more heat/energy used.
This is not quite the same as set richer, I'm assuming correct tuning.
The net effect is a slightly slower flame with each increase in nitro.Gas expansion still occurs slightly slower cooler but for longer.
Thus that soft wet buttery sound with high nitro fuel.
I hope this makes some sense.

Edit; That term that I could not remember to describe the combustion of fuels is 'Deflagration'.
Try to google; internal combustion and/or methanol and/or nitromethane and select the Wikipedia pages.

Brian