Cured Villager raising price over time

After curing a zombie villager, and only selling a bit of iron to them, they started to raise the prices of different items by 1 emerald every couple of days. Example The sale started as 1 emerald for and enchanted diamond sword and after approximately 9 in game days it’s now 3 emeralds for an enchanted diamond sword. The only thing I traded was iron, and made no other purchases.