You there, young man! Can you tell me how many boxes of missiles I am holding?

Uh, four?

Indeed! Now watch closely as I take them out and put them back in. Four boxes, once again. But what's this in my hand? There's a missile left over?

Ooh, can I have it?

Let's try again. Another missile! And where did this fifth box come from? Thank you, thank you, you've been a great audience, but I just violated a non-proliferation agreement so I'd better cut it off here. Goodnight everyone!

I just tried boxing and unboxing a bunch and didn't notice anything out of the ordinary. The boxing recipe turns 20 missiles into 4 boxes. The unboxing recipe turns 1 box into 5 missiles. The ratio is the same. These boxing/unboxing recipes are done by script, so they're all the same ratios.

One thing that's special about missiles is that they're ammo, so accounting for them could get messy if some go into your ammo slot automatically and some go into your inventory. They're also handcraftable, so it's possible if you had ingredients on hand you could wind up with an odd amount if boxing them accidentally triggered a handcraft of extra missiles to fill the box.

It happens when a productivity coprocessor is equipped. As far as I've seen, productivity increases the output of all handcraftable recipes.

I see. That's definitely an oversight. I'll fix it in the next update to only apply to recipes that allow productivity modules.

This is now fixed in the latest update.