They are correct! All you have is a very slight "overbalance" due to the lighter pistons. You didn't say what your bobweight is---probably in the 1600-1700 gram range. This 9 grams would be only 1/2% overbalance. Many high RPM engines use 1,2 or even 2 1/2% overbalance. Some shops overbalance 1% for everything but street engines.

What you don't want is an "underbalance" situation.