Make this Make sense

I changed my address information online today at Geico.com for my auto insurance, because I realized that I hadn't done that yet. So I get a lovely email response telling me this.

Your new six-month premium is $393.70.
This is an increase of $10.30 per six months.
This amount is prorated from today's date to the end of the policy period.
Your prorated increase will be $9.30.

How does changing my address from apt 80 to apt 82 increase my rate by 10.30 per 6 months? I don't understand...