Not sure where the figure 18.018018 comes from. The molecular mass of glucose is 180.15588 so, taking into account that a litre is 10 times the volume of a decilitre, the conversion factor become 18.015588. https://pubchem.ncbi.nlm.nih.gov/compound/D-glucose#section=Top
However, our measuring devices don't have that degree of accuracy so using 18 is fine.