On the subject of Expand Forests and why I think it gives economy slots.
We know forests do useful things to soil and rainfall patterns - see the entire arguments we've just had about the steppes and how we could invest to make them good for farming.
Economy slots indicate that there's things we could do to get more out of our territory - that some of it is going unused, or used inefficiently.
That more forests could thus increase the fertility or the amount of land we can cultivate -> more econ slots is an obvious conjecture.
And here's the thing: we've had anomalous extra econ slots show up before, after an Expand Forest.