I don't think they were striving for absolute realism concerning resources. To do so probably would have required the introduction of an overly-complex resource management system that wouldn't have been any fun for the average player to be confronted with.
The game tends to be slow enough in the latter stages of the game. Can you imagine how snail-like the pace would be if the game had to also calculate the precise amount of a resource each city consumed per turn?
I don't know about you, but having to peruse a stat-choked chart in order to ascertain the amount of oil I would need to import would have put me to sleep.