I guess because impregnable means stuff can't get in, while impregnated means stuff has got in.
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They both involve getting in but with different outcomes.
X
*has three blocks of extra mature Cheddar, a block of Red Leicester, a wedge of Brie, a tub of Mascarpone, two blocks of Wensleydale, a block of Cheshire, and a wedge of Parmesan*
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X