How can a GS Park not give my 10% off for extended stay?

They advertise 10% off the "nightly" park rates. The weekly, monthly, annual rates would not apply to the GS discount. That is unless you want to stay a week at the nightly rate with 10% off. I think the park discounted long term rates are a better deal. I see no foul here.
I am surprised the Rapid Response Team didn't point that out to you rather than form an investigation into the matter. I still think part of the RR Team are computer robot replies.