At the beggining of a natural stone/slate job I usually sort all the tiles into thickness piles. Start with the thickest tile and work down thicknesses, this way you can get a level floor. The thinner tiles can use a bit more adhesive but you should end up with a level floor.
Looks like the tiler has just thrown them down with a normal bed of adhesive.
Hope you don't mind MSB but i'm just going to quote you as this is exactly what i would have said.