That would not surprise me at all.  We were told that the small amount of blood from a finger or heel  prick has less plasma in it and more whole blood.  So I guess if they are measuring something like potassium, it is better to measure the quantity of that in a larger volume.  I have no idea.  All I know is that it is more accurate where measuring 17 OHP because of the reasons that I explained above which obviously has no reflection on the end result.  It may be a combination of  reasons I guess.