real cities don't typically produce food in real life, they may process food for shipping and what not but they do not have the food production capacity to support there residents. Food comes from the country side ie farms ranches orchards etc. so for a caravan to leave  a city without food and arrive at another city with food, I would assume it was gathered along the way by trading with the people that don't live in the city...  maybe that is why there is no extra iron, gold, or materials, gotta have some thing to trade for all that extra food from the poor peasant folk who live in the boonies     that's how i've explained it to myself anyway, would be nice to see them do more, guess we'll see in the next patch or two.

I felt the same way when I read that, I see a few others get it

this. exactly.

The good old Oregon Trail! though I only play the PC version (think it was version 2)

TBH really Caravans should produce a global resource boost in the cities they visit IMO, that right boost everything including Tech production.

I hate to say it, but I don't think there is any logic behind caravans yet.  I think they intended it to support trading actual resource types that the cities produced but they never got to it.  Instead we get this.  I have every confidence that we'll see this improved though.  As it is now caravans make roads and that's a good thing.