Suppose you have two oranges on the table. From your human, practical, primate perspective it makes sense to say that there are two oranges; your stomach has a limited capacity, and you may want to share the oranges with other primates; so you focus on the bits of information that are very relevant for your behaviour when fruit is concerned. A cockroach will only see a lot more orange that she can chew, perhaps too little rotten for her taste.
However, these perspectives (that are made of bearing in mind certain bits of information and becoming oblivious to everything else) are not reality. When a human being needs to make sense of the universe beyond oranges, he needs to take into consideration more relationships in the phenomenal world, and that implies a more complex analysis that the one required to count pieces of food, which is something by the way that even bees do.
If you say that there are two oranges on the table, you are actually saying: there are two oranges, and there is the relationship between them and the combined effect of the two as a system: the sum of their gravitational force; the increased probability that a fruit fly finds them (as the combined aroma is a stronger signal), the ideas in your mind about oranges; the perturbation of the electromagnetic field; and so on. All of these factors would be different or disappear if the oranges are put in different locations of the universe. So the location of the two oranges on the table at the same time, that is: the relationship between the two items, is also real, and physical, and an element to take into consideration. Therefore, you have two oranges, plus the relationship of the oranges between them and their environment. It so happens that what makes oranges oranges and not stones, are also relationships; so the difference between two oranges and two oranges and their effect on the world is just the number of relationships, or operations in the physical world that you are willing to consider.
As this always happens with any number of items, we can deduce that it´s never 2 + 2=4, but 2+2 equal 4 plus the effects derived from existing 2 oranges on the table and not any other number.
So when we say that if I eat three chocolate bars there is one left, what we do is: to consider only the levels of reality more meaningful for us (not the atomic level, where there are no chocolate bars; not the astronomic level where only massive celestial bodies are in sight); and you are actively ignoring all that is has to do with eating three chocolate bars and being one left. Which might not be much, or it might be a sick stomach; but in any case it´s never equal to zero. It never really is. Say that instead of chocolate bars, we are talking gun shots directed to you from a pistol which still has one bullet in its barrel.
You may say that all those effects are trivial, but they are not, because we live in a world in which the flight of a butterfly can cause major changes given enough time. That is, a universe where everything is connected. When we teach that two plus two equals four for real, we are encouraging people to be oblivious to many connections that are relevant for our problems. We teach the young to encapsulate their thinking process in disconnected boxes, and to lose the capacity to take into account factors that might change how we understand a problem entirely. We are not supposed to do that; we are not bees or chickens counting flowers or grains. We are animals that build whole worlds in our minds, to see many more connections through holistic images of the natural (and psychic) realities; even if we focus on one or two at a time when engaged in a practical action.