The point is that the _total_ number of trees is an astronomical quantity (roughly speaking, level 2 has 10 options, level 3 has 100, and so on almost the same way up to level 24). It is evident that 99+% are high-level branches, and only 0.0000000000000001 of the total is allotted to the first 8 (important to us) branches! Therefore, even considering that the authors changed this proportion — relatively few trees should still be assigned to the first options, roughly about 20 by eye, while for various Tazar types it's 5..7 for the first 3 levels, which allows Adelaide to get Diplomacy with a 90% probability.