What does the term "9g" in servo's rating mean?
According to Flite Test web site, the 9g reference is the weight without the wire lead. They measured one model at 8 grams rounded down. I suspect the weight also ignores the horn as well as the wire.

The article referenced on the web site calls the weight reference as a standard, which means that it more likely allows for heavier or lighter servos as long as the size of the servo matches. 

If you purchase a servo for a model known to use a 9g servo, you know the hole you have to build for it to fit. If you purchase a 5g servo, it will be a different hole and you may have a 6 or 7 gram weight, but the servo fits as it should.