D&D used to differentiate between trail rations and iron rations. The term “iron rations” dates from 1876, but it seems to have come to mean emergency military rations in the First World War. Both the Germans and English had iron rations, which were canned rations, not dried. English iron rations were 1 lb. preserved meat, 3 oz. cheese, 12 oz. biscuit, 5/8 oz. tea, 2 oz. sugar, 1/2 oz. salt, and 1 oz. meat extract, for a total weight of 2.2 lbs. per day. All of the trail rations in Pathfinder weight about half that. To compare, trail mix is typically 1/4 cup per serving (160 cal). 1 lb. of Planter’s fruit & nut trail mix has 13.3 servings.
The first edition Player’s Handbook (1978) listed iron rations for a week as 5 gp. and standard rations for a week as 3 gp. The first edition Dungeon Master’s Guide (1979) gives iron rations as encumbering as 75 gp. and standard rations as 200 gp., so back then the rationale was that iron rations were slightly more expensive, but weighed a lot less. This was obviously because iron rations were dried meat, pemmican, or trail mix, or some combination of these. So iron rations was dried or preserved food and would last longer (i.e. months) than standard rations, which might go back in a few days. This was further explained in 2nd edition, which said that iron rations consisted of dried meat, bread, vegetables, and some cheeses. Standard rations were “unpreserved foods, such as soft breads, fruits, vegetables, and fresh meat, in sufficient quantity to feed one person for one week.” Somewhere along the way, and I don’t know when, someone interpreted the dungeon to be a “mythic underworld” where standard rations spoiled in one day, and iron rations didn’t. Seems like a step too far to me.
Pathfinder has iron rations; it just doesn’t call them that. Instead, it calls them Halfling Trail Rations. Trail rations in Pathfinder weigh in at 1 lb. per day, unless eats Wandermeal (1 cp.) or Halfling trail rations (1 sp.), which are both 1/2 lb. per day. Wandermeal is not appetizing after a week with it, while Halfling trail rations are not only 1/5 the cost of normal trail rations, but also 1/2 the weight. I’ve got to imagine that Halfling trail rations are sold everywhere, because they are easy to make (dried meat, trail mix, cheese, molasses), inexpensive, and efficient. Elven Trail Rations is essentially trail mix, bread, and honey, but costs 2 gp. per day. That’s some expensive bread! Tolkien’s works used to be the basis of most D&D, and in his world, the men and dwarves used cram, while the elves had lembas. Both of those would have counted as iron rations.