I think the differences between of it is that developed countries refers to the sovereign state, whose economy has highly progressed and possesses great technological infrastructure, as compared to other countries than developing countries which is in process of their growth or is a country which has a slow rate of industrialization and low per capita income.
There are so many inequalities and differences between developed and developing countries in terms of the economy and the way of life of their citizens. You're right if the per capita income of every person in developed countries can reach $3000 every month just for ordinary employees, in developing countries only paid $80 per month, obviously the difference is..
The income difference is the main reason of why many professional from third world countries try to find jobs outside their countries, however that inequality in the payments does not come only from the difference in the size of the economies of those countries, many business in those countries know they can away with paying such low salaries because they know people are desperate to work and will do it for almost any price.