Villager trade prices vary by a large amount

After buying a ton of 1-emerald shields, my master-level villager upped their trade to 64 emeralds for one shield. This seems like a very drastic jump, considering one shield normally sells for 5 emeralds. The other trades the villagers offer also jump to 64 emeralds once I buy two or more items (In the images below, I bought the leggings and boots twice and the helmet and chestplate once – the items I bought twice jumped to 64 emeralds while the rest went up only slightly.)