When you go to "Options > Currency" the "Currency Digits" sets the number of decimals you're going to use, if you select $18.00 then only 2 decimals are going to be used, so if you enter 0.053, which has 3 decimals, only "0.05" will be saved as you selected to only use 2 decimals but are entering 3 decimals.
If you have been entering numbers with 2 decimals while using the $18.00 Currency digits option and then change it to $18.000, this change won't add a 0 to the decimals, it will set anything you have entered to have 3 decimals, so only the dot is moved as per your decimals number change, the program won't modify the amount entered, just move the dot as per the decimal amount you set.