It's in the DMG. Ability scores, or something. Scores start at around 13 (but primary attack score is 16), and then gain a half level. Perhaps it's in the chapter on NPCs, immediately after Customizing Monsters.
Hm, ok thanks - I found it on DMG page 184 'Monster Design Steps'. Ability scores ae not mentioned in the 'Monster Statistics By Role' table and they are unaffected by level up/down (page 174) so I never noticed this before! It says "Ability scores also help determine the monster's attack bonuses, ability and skill checks, and armour class" - but everywhere else attack bonus, damage, AC and NADs are simply a function of level, not derived from ability scores, so I never paid much attention to those scores.
So, after 3.5 years running 4e I've finally noticed that monsters are supposed to have a 16+1/2 level score in their primary attack stat and 13+1/2 level in their defence-setting stats, per DMG 184. If one of those 3-4 stats is Dexterity then the monster will have Init bonus = +level. Technically they could have Int-based AC & Ref, not have a Dex-based attack, and thus have an arbitrarily low Dex, though! Strange.
Edit: An obvious problem for having monster stats defined solely by level is that Minions get lovely high stats for their XPV while Solos have very low stats for their XPV. I noticed that running an adult green dragon last week that was a level 12 solo and only had STR 16, for a +9 Athletics check. Not keying the stats to the in-world reality (eg giants are very big & strong, but not too bright) seems to make them even more pointless.
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