When people started specializing in jobs, there were some jobs that there was tons of people working, but others where much less people were working/smart enough to do. For example, in the past there was tons of farmers, when there's so many of you, you don't make as much money because there is so much competition to sell you food. A job linked with farmers could be people who specialized in fertilization and such, that knew how to get more crops or better crops, and to keep the soil filled with nutrients. These people could create products like pesticides or chemicals that help your crops grow. There's less of these people than farmers, and the work they do is in good demand, so they make money, and would be higher in the social classes than farmers.
Another example is jobs that need you to be smart or very skilled in to do your work efficiently, such as chefs or programmers. Since there's a high demand for video games and newer and better tech, the best of the programmers or most creative make the most money, and programmers would fill the mid-higher social classes.