When assigned to a new duty station, why does a rental subsidy not cover the first 30 days?

Rental subsidy begins once you are settled with the DSA portion of the assignment grant covering the period when you first arrive. The subsidy is generally payable from the first day following the end of payment of the Daily Subsistence Allowance (DSA) portion of the Assignment Grant, or from the first day of your lease agreement, whichever date is later.