The rate of the salary is based on each employee's country of origin. Why? GCC countries like Qatar cannot offer lower salary to expat which are from countries which belongs to 1st world country because it would hinder those nationals to work. The reason for that is why they would work abroad if they can earn better in their homeland given the fact that you will adjust with Qatar culture, climate, working enviroment, and lifestyle where in your own country you can fully enjoy and theres no adjustment needed.
If you are from 3rd world country which you are deprived to have a better pay or chances of unemployment will take whatever Qatar could offer rather than inadequate salary or worse face unemployment.
Simple as that.