Yeah, I think those Grandfathers were on a whole other level of might, than the ones in Eluria. So that makes sense.
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I think that the 'grandfather fleas' show up in yet another King book, 'Needful Things'.
It's mentioned that the necklace Polly gets from Gaunt makes insect like sounds and such...it could well contain a grandfather flea. And our man Gaunt does get around. He could easily have picked one up somewhere.