Perhaps the rationale behind this ruling is that it takes time to get accustomed to the officer lifestyle - the fine wines, trips to the opera, guns and dogs - and to leave the beer and sandwiches of the Sgt's Mess behind. Therefore, the expenditure is not incurred immediately and the extra dosh is not required!
BTW, before I am bombarded with hate mail, this is a joke. Probably not to PTC though!