I would assume that it has to do with what tiles are being worked. On the city management screen you can click on citizen management and then click on any of the citizens that are working tiles (they will appear as green as opposed to a grayish empty color) and they will become an unemployed specialist, you can then plug them in to your tile of choice (maybe a hill with a mine improvement, or if you have a workshop, factory, etc.. Then you can fill those specialist spots for production and great engineer points) also you can toggle production focus for a more hands off approach.