Did you know that the conversion factor of 18 is specific to glucose? The conversion by 18 essentially says this: 10mmol/dl = 18 mg/dl. 10 mmol of sugar = 18 mg of sugar. A mmol, a milimole, is 602,000,000,000,000,000,000 molecules.
6,020,000,000,000,000,000,000 molecules of glucose weighs 18 mg.
That is why, when you look at converting something else from mg/dl to mmol/l or back (say, cholesterol) you'll have to multiply or divide by a number that's not 18 (in the case of cholesterol, 38).