My question is whether all the excess energy that is left every day always go to fat, or can it be wasted or disposed of somehow? (for example by not digesting everything to 100 percent?)
Now, I am not asking for "ways" of consciously doing this, I understand that it can be difficult, but physiologically speaking, do any such disposing processes exist?

That is my main question, but just for some more clarity:

Let's call all the energy chemically contained in a day's food A.
Let all the energy received by the body from digesting that food B.
Is A = B? If B is less, is it always smaller than A by some sort-of constant factor, or does it depend on anything going on in the body?

Further, let C be all the energy used up by the body for "real work" like driving muscles, sending all the electrical signals in the neurosystem, running all the chemical reactions that require energy in the cells and building new cells.

Now, let's say that any given day C is less than B. So now, my main question explained in variables would be: any given day, does all the excess energy (B-C) all go to fat, or can a part of it be disposed of in some way? If so, how? (again, not "how do I do this?", more "what physiological processes are going on if this happens?".)

It sure seems that way because people seem to have very different metabolisms, some never get fat no matter how much they eat (not only can their metabolism burn a lot, but it is *ADJUSTING* to their diet) , and others gain weight very easily with just a little excess energy.

What are the physiological mechanisms behind this?

-- Update --
Thanks for your answers, I will try to clarify the point of the question.

Basically, the point is: "Physiologically, how do some people seem to keep their weight no matter how much they eat? (They keep the same weight if they eat a lot, BUT ALSO if they eat normal amounts). Because them doing so means that the rule "All excess energy that is digested but is not used up goes to fat" *does not apply*. For them the excess does not seem to go fat, so where does it go instead?" 
For example, see links in this answer to another question: [No such thing as a hardgainer?](https://fitness.stackexchange.com/questions/1934/no-such-thing-as-a-hardgainer/1937#1937) 
{label "MAIN QUESTION"}

I see only 2 ways of this happening, and I will keep using my variables, even though they are not 100%-bulletproof of course, considering such things as body digesting its own cells, thus sort-of increasing B without increasing A; C being used up for digestion (increasing B), and so on. 
Think of variables as just a way of explaining the question and having a general mathematical picture of what is happening. Also note they only count the energy, not vitamins/proteins/etc.

Either 
1) Any given day, ratio of A/B is sort-of the same, and depends only on a) generic personal metabolism (doesnt change much from day to day), b) that day's diet c) maybe some secondary random factors such as subject's stress level or so.
This way, if B>C, there must be some mechanism in the body that can use up arbitrary amounts of energy for doing same thing, and it is using more energy if the people in question eat a lot, and is using less if they eat normally. 
What is this mechanism and how does it know how much energy to use? Any specific hormones doing this? Or is it building some chemicals instead of others?

2) Any given day, if there was a lot excess energy (B-C), the digestion process for the next day is regulated in such way that the next day (or "period of time", I don't think our body plans in days, but just used for clarity) the A/B ratio gets higher, thus the next day's B will get lower, and this way (B-C) of the next day will be lower or even negative, thus preventing the building of fat. Is this the case? Are there any specific known hormones/mechanisms that regulate the digestion this way? (dependent on the body's general energy vector (excess/deficiency)

Or I have some very big logic flaw somewhere. In that case please just answer the part labeled {label "MAIN QUESTION"}