Markets and trading rates

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This is all a bit confusing but appears to be a bug?
Ore market - doesn't appear to upgrade past level one, best rate 5:1
Lumber market - doesn't appear to upgrade past level two, best rate 4:1
Wool market - doesn't appear to upgrade past level three, best rate 3:1.
The card description says it should improve trading by 1, so in theory the best we should get is 5:1 without a farmers market.
Should these buildings even be upgradable?
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Comments
Sorry, looks like I can get 4:1 on everything, but wool is 3:1. No farmers market built.