I noticed some resource combos are very common while others are not.
Very Common:
Rubber + wood
Pigs + Cattle
Pretty common:
Uranium + Lumber
Uncommon:
Gems + Water
Gold + silver
Bullets + Uranium
Is the game designed to make some combos common? Or is it because of re-rolls? I can understand why re-rolls would make some combos very rare, ie gems & water. But to make some combos far more common than others seems statistically impossible... Do most people re-roll 10 or 20 times?
I ask this because I have one of the common combos, and therefore lots of competition; it is hard to find a trade with someone that doesn't already have my resources...!