There are multiple effects, tamper material is only one of them. Ground bursts also increase the overall amount and concentration of fallout: First, with the e

There are multiple effects, tamper material is only one of them. Ground bursts also increase the overall amount and concentration of fallout: First, with the explosion closer to ground, more neutrons can reach the ground and activate material there, producing radioactive isotopes from previously harmless earth. The "fireball" is a shell of expanding lower-density plasma which doesn't dampen neutrons as much as gas (especially water vapour) in non-plasma air. So that fireball touching ground produces more activated elements increasing overall fallout. Second, all newly created active elements and more fusion/fission products get bound to ground particles like dust, coral or water droplets if the explosion stirs up more of that. Those particles are larger and heavier, producing a more concentrated fallout closer to ground zero, whereas unbound products would be dispersed higher in the atmosphere without producing too much of a distinctive fallout pattern.