The total length of the wire used to make the coil determines the resistance. All wire has some level of resistance per mm or inch. In addition to the coil itself there's more wire in the legs that connect the coil to the atomizer screws. That adds a minute amount of resistance to the overall ohm rating of the coil. The coil winder algorithms are just taking the entire length of the wire into context to more accurately calculate the total resistance. It won't be much but it can be the same as adding an additional turn or two to the coil.
I leave mine at the standard 5mm setting that's included in the coil winder calculation. That accounts for 2.5mm legs on each side of the coil going to the screws. Depending on whether you orient the coil parallel or at right angles to the posts, there might be more than 2.5mm on each leg.