that is because the more you wash them, the worse they smell. i NEVER wash my ferrets unless they get into something. when you bath them, you strip their skin of essential oils, so their body goes into overdrive to replace them, which makes them smell worse.
to the OP. Descented, just means that their scent glands have been removed, not that they won't smell.
if you want to avoid smell, DON'T bath them. they don't need it. DON'T feed them the crappy Marshall food. its main ingredient is fish, which makes their poo smell 10x worse.
i've owned Ferrets for 15 years or so now.