Companies that use Mud or Dirt to raise the concrete are assuming you are ignorant.
NO reputable contractor grades out dirt and pours concrete over it. They ALWAYS use a Roadbase product or Gravel. The Aggregate is needed to hold the material in place.
When you just have dirt, the moment it gets wet it moves. Prove it to yourself. Drive out into a farmers field when it's wet and try not to get stuck. If you drive out on gravel road you won't have an issue but on dirt, you will.
So why would anyone put mud or dirt under a sunken concrete pad?
We have some competitors who claim they add "6 bag concrete" to their mud or dirt mix it somehow increases the strength.
Again, they are assuming you are ignorant.
Concrete does not Bond to Dirt or Mud. Jack Hammer out a piece and you will see.
These companies use Mud or Dirt because it's cheaper. Also, they don't like pumping the Aggregate because it's harder on the machine, not to mention most don't have a machine capable of pumping a Gravel Slurry like Concrete.