A wool block fuels a furnace for 5 seconds, so 2 wool blocks are exactly enough fuel to smelt a single item. 2 wool blocks also craft into 3 carpets, which at 3.3 seconds each are almost enough fuel to smelt a single item, but not quite. This is most likely a rounding error, since 1/3 can't be represented as a floating point number.
- Put a smeltable item (e.g. iron ore) into a furnace
- Add 3 carpets as fuel
- Wait 10 seconds
Suggested fix: Make a carpet fuel a furnace for 3.4 seconds instead of 3.3. This would make carpets slightly more effective than wool blocks, which seems acceptable considering wood and planks are both worth 15 seconds of fuel, despite one log crafting into 4 planks.