Ores doesn't generate with cluster size == 30 even with scarcity==1

[4aiman]

Reproduce steps:

• create new mod;
• add this to the init.lua:
  minetest.register_ore({ ore_type = "scatter", ore = "default:stone_with_coal", wherein = "default:stone", clust_scarcity = 1, clust_num_ores = 27000, clust_size = 30, height_min = -1000, height_max = 0, });
• create a new world using mapgen v7;