You are looking at ADHD as what the signs are and even there, you are missing most of them. There are a lot more symptoms than signs in ADHD the inability to focus is just one of many.
But more important than that is what ADHD is neurologicaly. I simplified it a ton to make it easier.
An imbalance is an incorrect amount. There are normal ranges that humans fall into. Some more, some less , that's why it's a range. The body works properly when it falls in that range. When we don't have enough or too much of something our bodies stop working they way they should and results in disorders. That's why hospitals will draw labs and see what is elevated, or depressed. We know what the general range should be, so we know when something is too high or to low.
This has nothing to do with sitting behind a keyboard and everything to do with faulty wiring and suboptimal amounts of norepinephrine. Not to mention that the brains of people with ADHD have a smaller frontal lobe and noteable differences in the limbic regions and basically ganglia.