Well, I'm not sure if the CR table is quite right. The samples look better than their CR, I think. I think I'd push the CRs in the table up a step each to be a little more comfortable with it.
Yes, you're on to something there. I eyeballed them as CR2 with my first rough draft. Part of the problem is that HD alone are not a good basis for judging zombie nastiness, since AC and attacks are also important.
The Wyvern is probably not a good basis for comparison, since it has lots of attacks. Consider a Dire Wolf Quick Zombie:
Dire Wolf Quick Zombie
Large Undead
Hit Dice: 12d12+3 (81 hp)
Initiative: +4
Speed: 80 ft. (16 squares)
Armor Class: 20 (-1 size, +4 Dex, +1 dodge, +6 natural), touch 14, flat-footed 15
Base Attack/Grapple: +6/+18
Attack: Bite +13 melee (1d8+12) or slam +13 melee (1d8+8)
Full Attack: 2 bites +13 melee (1d8+12) or 3 slams +13 melee (1d8+8)
Space/Reach: 10 ft./5 ft.
Special Attacks: Haste
Special Qualities: Damage reduction 5/slashing, darkvision 60 ft., undead traits
Saves: Fort +4, Ref +9, Will +4
Abilities: Str 27, Dex 19, Con —, Int —, Wis 10, Cha 1
Skills: —
Feats: Toughness (B)
Environment: Any
Organization: Solitary, gang (2-12), or mob (10-20)
Challenge Rating: 5
That's not as tough as the Wyvern but is still more like a CR6 than a 5.
Perhaps we should use the skeleton's CR progression instead, with the CR tweaked up by one?
'HD ' ' CR
1/2 '' 1/3
'1' ' ' 1
2-3 ' ' 2
4-5 ' ' 3
6-7 ' ' 4
8-9 ' ' 5
10-11 ' 6
12-14 ' 7
15-17 ' 8
18-20 ' 9
What thinks thee?