"Robber camps spawn only on fields with 7c and 3456 distribution."
Is this really in the code, or some kind of strange superstition ?
The fact you're asking this proves you've never even joined in a decent kingdom that was tight for space.
These camps spots should NEVER be filled, since then there would not be any camps spawning. It's a dumb system, but sadly kings have to make sure governors know these rules.
A general rule is that each treasury spot (city influence space) should have 4 robber spots (7c or unbalanced) in them. The amount of robbers that spawn is paralel to the kingdom size. It's possible all camps spawn on the east side, even if there are suitable spots in the west side. This is just random, but if you leave enough spots open, you at least make sure you get all camps your sized kingdom should have.