Umm, on that chart, the WotC conversion rules were not used right, especially on "to hit" mods.
Firstly, the "To Hit" column is apparently being calculated as HD/2 - 2 when it's supposed to be HD/2 + 2. For example, the Bugbear should have +4 to-hit instead of -1 (it also looks like he was rounding all division down instead of nearest whole number.
Second, the Dex Mod conversion left it up to the DM whether a bonus was appropriate or not. The first 5 entries look similar to each other since they would all conceivably be dextrous, but the next 5 shouldn't have any bonus which would put them very close to the 5E versions. The guidelines also said some monsters should have penalties if appropriate, so a DM call of converted Zombies having a -2 Dex would be likely.
Third, the damages aren't looked at in any detail. In 5E, ghouls only get 1 bite (9 dmg) or claw (7 dmg), while in 1E they get 3 attacks (claw 1-3, claw 1-3, bite 1-6) that average to 2+2+3.5=7.5 points of damage, functionally identical to the average of 8 that they deal in 5E.
Hit points and damage however do scale quite differently between editions, that's true. As others mentioned, that could be a feature instead of a bug when it comes to running AD&D adventures on the fly. If you wanted to do a more faithful monster conversion to use in your own adventures, doubling the 1E hps and damage dice would go a long way. A more detailed HP could be to roll or take the average 1E hps, then for Large creatures add +2hp per HD, +4/HD for Huge, +6/HD for Gargantuan, then double that total. For example, that would give Ogres 60hp and Owlbears 63.
A note about the Green Dragon: 1E dragons are rather puny compared to future editions, even 2nd. Converting a 2E Young Green Dragon works a lot better:
- 1E: 50hp
p), 18AC, +8 to-hit, 3 attacks ((8)1d8+3, (8)1d8+3, (14)2d10+3), 27 breath damage, +0 Dex mod
- 5E: 136hp, 18AC, +7 to-hit, 3 attacks ((11)(11)(22)), 42 breath dmg, +1 Dex
- Extra HP and Damage: ((4.5 + 2) * 11) * 2 = 143hp, double damage dice: (12)2d8+3, (12)2d8+3, (24)4d10+3, 48 breath dmg
---
EDIT: For my own methods, if you're using fixed HP and/or Damage, a better way could be to just take the max 1E HP then add the +2/4/6 per hd for L/H/G size creatures. For damage, do max 1E damage, then add a PC equiv. proficiency mod to damage (HD 1-4 = +2, 5-8 = +3, 9-12 = +4, 13-16 = +5, 17+ = +6), which would reduce damage a little vs. doubling dice. The dragon above would change to 110hp (for large) / 132hp (for huge), and damage would be 12, 12, 18, 46 breath.