| TempusAvatar |
If you look at Table 3-8: Animal Companion Base Statistics on p.52 in the Core, you will notice that "Class Level" and "HD" do not correlate evenly. However, if you compare "HD" with "Special" you will notice that those numbers do correlate. The animal companion receives the ability score increase every 4 HD.
P.S. I also missed that class level and animal companion's HD don't correlate evenly (until I studied it closely.)