furnace counting incorrectly

These are normal rates.
For more information:
https://rustlabs.com/item/furnace#tab=smelting

https://rustlabs.com/item/large-furnace#tab=smelting
Hope this helps to eliminate the confusion. 

~G

forlarge furnace?

cant be adding correctly there was a slightly different amount every furnace

and i put in same amount i each

It's not an exact science. Most times it's averaging a 75% return from the wood provided.  That applies for large and small.