"hammer decay" seemed to be the trick, thanks. I found this:
The modifier BUILDING_PRODUCTION_DECAY_TIME has a value of 50, probably meaning that building production starts decaying after 50 turns.
The modifier BUILDING_PRODUCTION_DECAY_PERCENT has a value of 99, probably meaning that building production decays at 1% per turn.
The modifier UNIT_PRODUCTION_DECAY_TIME has a value of 10, probably meaning that unit production starts decaying after 10 turns.
The modifier UNIT_PRODUCTION_DECAY_PERCENT has a value of 98, probably meaning that unit production decays at 2% per turn.
Here:
http://forums.civfanatics.com/showthread.php?t=142345&page=2
It leaves two questions open:
1) Is it total turns not worked or most recent turns not worked (Esako, I imagine you are right and that it is total number un-worked - makes sense to me, but couldn't find confirmation).
2) Is the percent mentioned a percent of the total required hammers or of the hammers worked so far (my suspicion is its on required).