Why do people compare their intelligence as if it's a number?
Part of it is the restraint of language. We only have two words - smarter, and dumber. Telling someone that you are "smarter, but of course only in x, y, and z" is too much work. And yet it's much closer to the truth than leaving it at "smarter."
Some people are better at some things than other people. More importantly, it's not linear. Steve Jobs may have been paid 100x more than the average Joe, but do you think you could have replaced Jobs with 100 Joes, and still created the iPhone? Interestingly, you probably couldn't have replaced Joe the plumber with Steve Jobs, either. The smartness-as-a-scale model is breaking down.
Here's a better approximation for your intelligence. Imagine a shape, like a blob or a five pointed star. The bulges of the shape show where you know "more" about something. Notice that comparing Steve Jobs and Joe the plumber is rightfully ambiguous here. Also notice that we can't ever really describe your full intelligence - we'd have to add more and more topics, so the shape would get bigger and bigger.
With technology and plumbing, the trap of comparing apples to oranges is easy to see. But it quickly gets difficult to tell, especially when English words don't point it out for you. Your strength vs. his strength? The word is the same, but it depends on what you mean by strength. You would have to zoom into the shape, adding the amount you can lift, endurance, etc. until you've approximated the meaning of strength. It's not likely that one person is overwhelmingly superior to the other.
When I get in a deep discussion with someone, our intelligences are being overlaid. We each have edges that the other doesn't. That's what makes them interesting to talk to. They can surprise me. And both of our knowledge pools get a little deeper, but in different directions. They start to match better. Perhaps this is why you might be considered, anecdotally, "the average of your five closest friends." The deeper the discussion, the more of the shape of each intelligence is revealed. That's how best friends know each other so well.
The "directions" of your intelligence might be why comparing based on the "total area" is so dangerous. It would be like comparing a hammer and a screwdriver based on their weight.
Some shapes completely encompass others. Kids, for example, often think adults are "all-knowing." How could they not? Dad seems to know more about everything.
But the situation is rarer than you might think. You would be hard pressed to find someone who doesn't know something you don't. I'm not talking about facts - facts are just noise around the shape. Someone has probably thought about something much more deeply than you have. After all, you have a limited amount of time to think about things. It would be an incredible coincidence if you had already independently considered everything the other person had.
Is it possible to stop comparing yourself to others? Probably not. But it is possible to avoid comparing an apple and orange, while mistakenly thinking that you have two apples. And that's worth thinking about.