All villagers contains the trade with Rotten Flesh and then Gold to exchange one Emerald but the first one exchange 29 Roten Flesh versus 1 Emerald and the second one 27 Rotten Flesh versus 1 Emerald.
I have tested this bug in 5 differents villages and ALL villagers have this bug !
Thanks for correct this bug and for the reading.
WAI, the price of trades varies between villagers.