Scoreboard Objectives Inaccurate in Smelting Objectives

What I Did Before the Bug:

Before the bug actually happened, I have set a scoreboard objective so that a player's "score" will be raised whenever somebody smelted iron, with this command:

/scoreboard objectives add ironCrafted stat.craftItem.265 Score (Iron Smelted)

Then, I have displayed it on the sidebar:
/scoreboard objectives setDisplay sidebar ironCrafted

What I Expected to Happen:
When I was smelting iron, I expected the score to raise accordingly to how much iron I have smelted.

What Really Happened:
Whenever I take out some iron in bulk (say, I take out a stack of iron ingots from the furnace), my "score" of Iron Smelted increased only by one or two.

How to Reproduce the Bug:
1. Use these commands to set up the scoreboard:
/scoreboard objectives add ironCrafted stat.craftItem.265 Iron Crafted
/scoreboard objectives setDisplay sidebar ironCrafted

2. Then, smelt a stack of iron ore. Wait for the majority of it to be smelted.

3. Take the iron ingots by hand. The scoreboard will only raise by a few.