stacks of 65 when emptying furnaces

What I expected to happen was...:
for me to shift click the furnace and get 64 ingots of gold/iron

What actually happened was...:
every 1 in 5 stacks-ish i would get an extra ingot stacked on

Steps to Reproduce:
1.smelt 64 ore
2. add more part way done
3. while furnace is on shift-right click the ores out