I remember Alex saying that custom production should remember the credits you don't use every month so the player isn't compelled to max his production budget every month, but while playing it seems that unspent production is wasted.
Hmm, I don't remember saying this; if I did, I must've misspoken.
What happens (well, not any more) is if you have a large ongoing project (such as a battleship) that exceeds the monthly capacity, the monthly production gets applied towards it. Back when this probably came up, the production cost would be deducted in one lump some on project completion, which might explain what you're remembering. Now - in the current release - this ongoing costs gets deducted monthly, to make it clear what's happening.