Villager Trading: The Buy Numbers Change

This is a rare bug I found in minecraft. For example, the villager offers me 12 cooked fish for 1 emerald. I trade with him once and I exit the trading menu. When I right-click on him again, things changed. Now the original trade become 9 cooked fish for 1 emerald. That means the number of buy changed automatically. This will also happen to every trades(I though). I encounter once more with 16 wools for 1 emerald and it changed to 15 wools automatically. I can't provide any image because it took randomly. I don't know will this help but thanks for reading!