Math is an abstraction, it isn't real. Thus math relies on assumptions, some of which are treated as real for simplicity's sake. You could say if you have one apple on a plate and you add another, then you have 2 apples. That's functionally and understandably accurate, because we assume all apples are the same, and the space they occupy is their common ground. Now let's use math to tackle something a bit more complex like turbulent boundary layer of an aircraft under yaw, sure 1 + 1 still equals two, and high pressure seeks low pressure, but how do you use that to make a better wing with less drag or more lift? You can't 1 + 1 your way to an answer even if you use a super computer cluster. You just have to test and see what works and what doesn't, and if your design doesn't work it's a very costly mistake involving lives and millions of dollars of plane. In engineering, the specification is always +/-, there's always a tolerance. Things are considered functional, or serviceable if they fall within certain tolerances, just like the temperature in your body.