It has to do with the probability of a given die roll occurring. There are three ways to make a sum of ten with a pair of six sided dice:
6-4, 4-6 and 5-5
There are five ways to make the sum of eight (ditto for a sum of six).
6-2, 2-6, 5-3, 3-5, 4-4
So you are almost twice as likely to see an eight rolled as a ten. The game actually has little dots underneath each number showing how many possible die rolls produce that number (so an '8' has 5 dots underneath it, a '12' has just one).
Seven is the most common roll, but it has a special meaning in the game and does not correspond to a resource hex.
For balance reasons you don't want a settlement location to have a super high probability of producing resources. So you don't let '6' and '8' sit next to each other. Otherwise the player with the settlement spanning them would likely win easily.
6-4, 4-6 and 5-5
There are five ways to make the sum of eight (ditto for a sum of six).
6-2, 2-6, 5-3, 3-5, 4-4
So you are almost twice as likely to see an eight rolled as a ten. The game actually has little dots underneath each number showing how many possible die rolls produce that number (so an '8' has 5 dots underneath it, a '12' has just one).
Seven is the most common roll, but it has a special meaning in the game and does not correspond to a resource hex.
For balance reasons you don't want a settlement location to have a super high probability of producing resources. So you don't let '6' and '8' sit next to each other. Otherwise the player with the settlement spanning them would likely win easily.