To be honest, there are a number of factors that play into resource extraction. Many of them are quite complicated. Consider the following examples.
Province A produced 10 units of iron in 1936 and 100 units of iron in 1996.
Province B produced 10 units of iron in 1936 and 100 units of iron in 1996.
Province C produced 00 units of iron in 1936 and 100 units of iron in 1996.
Province D produced 100 units of iron in 1936 and 00 units of iron in 1996.
Province E produced 10 units of iron in 1936 and 10 units of iron in 1996.
Province F produced 10 units of iron in 1936 and 100 units of iron in 1996.
Province A: Province A is in the middle of nowhere and there was insufficient infrastructure in place to allow for increased production in 1936. Some rail lines, roads, and a canal allowed it to increase production.
Province B produced iron of inferior quality until new technologies in iron extraction increased the effective output of the mine.
No one knew province C had iron until after the war was over.
Province D was a prosperous iron mine, but it eventually went dry. While it could technically produce some iron in 1996, no one feels it is worth the cost.
Province E has seen increases in infrastructure and efficiency, but it's also slowly running out of good veins offsetting the production gains.
Province F is in an area that was sparsely populated until well after the game's timeframe; only when it gained population did it also become a hub for iron production after people living in province D moved there to get mining jobs.
These are just the things I could come up with in a few minutes. There are countless other reasons production to change in a province. Infrastructure isn't the only factor at play.
I've played a game that had some of these factors in play, and it's called Vic2.