Mike,
Filthy upholstery that has been dampened on the surface will wick all the dirt that is in the sub-surface, be it, calaco wadding, felt, foam, sponging.
Upholstery actually sucks dust, smoke and fluid leaks further and further into the soft cushioning as people sit and raise.
If you were to put some clean white cotton over a filthy dry sponge, then spray the surface of the fabric with water, the dirt will wick and turn the cotton brown. The more you spray the cotton the worse it will get.
Cleaning only the surface fabric is not enough in most commercial cases.
Pete