This link will let you input what you have and tell you exactly how much and where to put the lead to get a desired spec.
The reason for the difference is that we post the average strung spec of around 6 racquets so it is unlikely you will get an exact match. Spec tolerances are usually around:
Weight: +/- 10 grams
Balance: +/- 10 mm = 3 to 4 points
SW: +/- 10 points
Ra Stiffness: +/- 3
Hope that helps.