Difference between Tavern and GEM SHOP

I Want to buy a 3rd General, 3rd Explorer and 3rd Geolist.
I can do this in 2 ways: Tavern and GemShop (Earned GEMS)

When i buy them from Tavern they costs:
General - 500 coins
Geolist - 600 coins
Explorer -1600 coins

When i buy them from Tavern they costs:
General - 1250 GEMS
Geolist - 300 GEMS
Explorer - 995 GEMS

When i put them in one line you get
General - 500 coins General - 1250 GEMS
Geolist - 600 coins Geolist - 300 GEMS
Explorer - 1600 coins Explorer - 995 GEMS

When i calculate the value of the GEMS they are worth
General - 1 GEMS = 0,4 gold coins
Geolist - 1 GEMS = 2 Gold coins
Explorer - 1 GEMS = 1,6 Gold coins

Why the different values ?

The price in the tavern goes up the more you get. So you can't compair them like this.