You are asking what the difference is between someone who has learned the tricks of the trade, and someone who has not learned the tricks of the trade. Would you expect an auto mechanic who was totally self taught to be as good as one who was factory trained? That's probably a bad analogy, with car repair, it's either right or wrong. With upholstery, there are shades of gray. I'm not talking about ability, or experience, I'm talking about knowing 10 different ways to do the same thing and have it come out just as good all ten ways. I'm also not saying that you can't be a success if you're self taught, there are lots of people out there who do fabulous work and are self taught.
I have almost 40 years of experience, and I believe that you can't learn upholstery by reading a book, even if it has lots of pictures. For one thing, different areas of the country refer to things differently. There are also no two upholsterers who do everything the same way. We all have developed our own ways of doing things. At some point you need someone standing beside you to guide you. Why not find someone at an upholstery shop that is getting ready to retire and ask them to mentor you? Don't bother asking a younger upholsterer,.........nobody will train their own competition.
No one lives forever, the trick is creating something that will.