When you have Looting on your sword, what all we knows, it's had a probability who the mob you are killing drops you more items. When you use Fire Aspect in the sword, obviously, the mob set on fire, but, when you have Looting and Fire Aspect on the same sword it's changes.

I mean, if you kill the mob with the last hit with Looting it drop more items, But, if the Fire Aspect kill him, it doesn't drops you more items, if you know what i mean.

I don't know if this is a bug or just the gameplay.

Cheers.