Wow the math adds up and the author is very consistent. If we take their stat total (S), assume they start at lvl 1 opposed to their current level (L), and also assume they start at level 1 with 1's across the stat board (base stat total 6), we can make the formula (S-6)/(L-1) to get the stat growth per level up. Thief's stat total is 32, archer's is 68. Plugging them in, we get Thief = (32-6)/(14-1) = 2, and Archer = (68-6)/(32-1) = 2. Every level up they get 2 points, either at randomly added or as free points.