I was trying to ratio-out a small iron plate making factory. I used this calculator which indicated that I need 0.7 electric mining drills in order to feed 1 stone furnace:
This is equivalent to saying I need a mining drill running at 70 percent speed to feed 1 stone furnace (you can't reduce the speed of anything in Factorio though). A stock electric mining drill can harvest 0.5 items a second (or 1 item every 2 seconds, or 30 items a minute). A stone furnace can smelt a single iron ore in 3.2 seconds, thus 18.75 iron ore a minute. But, "0.7" electric mining drills means it should harvest 21 ore a minute, yet this calculator says 18.75. If I did my math right, I should only need .625 of an electric mining drill since 18.75/30 = 0.625 items a minute.
Are belts being factored in somehow? The calculator also shows belts, but I am not understanding how they are being factored in exactly (if at all). The yellow belts can move 900 items a minute (15 a second).
Note: I am ignoring the coal portion of this. It shouldn't matter anyway for my question.
Why do I need 0.7 electric mining drills instead of 0.625 to feed 1 stone furnace in order to make iron plates?