Look at the rate of feat acquisition in 3.5 D&D. You get new feats at levels 1, 3, 6, 9, etc. What's wrong with that? There is a two level gap between 1 and 3, and a three level gap between all the others. That's not elegant. Moreover, if all levels should be equally valuable, then levels at which boosts are granted should be spaced equally apart.
Ability boosts, on the other hand, are gained at levels 4, 8, 12, etc. That's better, right?
Not really. When you put the two progressions together, you get "good" levels at 1, 3, 4, 6, 8, 9, 12, 15, 16, 18, and 20, over a 20-level career. But that makes levels 2, 5, 7, 10, 11, 13, 14, 17, and 19 "empty" or "bad." And it makes level 12 "supergood," since you get both boosted then. Ugh.
Look, everything else your character gets in 3.5 Dungeons and Dragons -- hit points, attack bonuses, saves, class abilities -- come from your class. If all class levels are equal (and they are not, but if they were, as they should be), then the other things you use to improve your PC (feats and ability score boosts) should also be equal.
To achieve that goal, in Scurry feats are gained at levels 1, 4, 7, 10, 13, 16, and 19. Ability boosts are gained at levels 3, 6, 9, 12, 15, and 18. But that still leaves "empty" levels. Therefore, in Scurry, you also gain race-affiliated boosts at levels 2, 5, 8, 11, 14, 17, and 20.
For example, a cockroach PC gains bonus skill ranks at level 2, +3 hp at level 5, more bonus skill ranks at level 8, +1 natural AC at level 11, more skill ranks at level 14, +1 on all saves at level 17, and more skill ranks at level 20.
In this way, outside the traditional class structure, so to speak, a character gains something useful every level. No one level offers more than any other.
