Linden Lab Official:Did you make a mistake when you billed me for my Mainland land holdings?

On the Mainland, we bill based on peak usage for the prior 30 days; your land's billing is based on the date the land was acquired, and the amount of land held in the prior 30 day period.

Our Land Pricing page states this as:

The Land Use Fee is a monthly charge for the peak amount of land held during the previous 30 days, including actual parcels held and land tier donated to groups.

Once the peak usage has been billed, your peak usage is reset to the current amount of land you own or have donated to a group. All land owned or donated during the prior 30 days is counted, no matter the amount of time it was held.

Account fees, which are separate from mainland billing, are based on the user's sign up date and are paid in advance.