This is because they are "levelled up" using NWN internal NPC level up system.
This mean it has to use "packages" which are built using a 2da, which is a horribly annoying method to modify things with.
If you want to make a load of specific packages for these creature, go for it! we'd love the help.
However, I did make a "level down" system more recently, which doesn't require these packages, but it's still not fully ready for use with Ranger Animal companions. Once I get the skills and stat points being modded properly, this might be a better method to use with these, as then we could just make the creature at the highest level possible, and then level it down to the ranger. Removing things is a lot easier than adding things, as, how do you know what to add? (without a packages.2da?) whereas you can know what to remove somewhat, as there can only be X feats at X level, some have prerequisites, you can only cast so many spells etc etc.