This has less to do with a "fairness" basis and more to do with logistics. Simply put, rear-area units, typically being deployed closer to the supply lines and near main roads, away from the combat zone, were much easier to supply than the front line, where combat damage, entrenchments, and bad terrain would complicate logistical and resupply efforts. Often times when going into combat, troops would bring designated combat rations, which while light and portable were typically inferior to the food soldiers would receive in the rear. For instance, in the case of the Imperial Japanese Army had several types of rations, delineated between ones in garrison (where they typically had more freedom to procure various kinds of food), ones in the field (around 4.5 lbs of food), ones specifically for combat (around 3lbs of food), "emergency" rations (around 2lbs of food) and finally "iron" rations (an assortment of high-calorie foods like biscuits, rice crackers, and the like). Of course, in the case of the Imperial Japanese Army, logistics and resupply were a constant issue and troops often received substantially less than the stated amounts, and soldiers often ate poorly, grass, or not at all.

Source: TM-E 30-480, US War Dept.