Which is more reliable? Measuring with the jar provided or with a medication syringe?

I am having trouble measuring a medication for my child. Since it is difficult to find advil and tylenol for infants, I give the liquid version for children to my 14 month old daughter. I respect the quantity calculated by the pharmacist. I use the little jar that comes with the syrup to measure. To make it easier to give to my daughter, I then take it with a medication syringe (I had tylenol and advil for infants). I noticed that the amount is always smaller in the syringe. For example: I measure 5 ml with the little jar, but when I put it in the syringe sometimes I have barely 4 ml. I'm afraid of not giving the right amount (too much or not enough).