The designers didn't use their own guidelines. This is most obvious with damage values, but if you take a close look at attacks and defenses you'll realize that they also ignored their own formula.
If you're interested, I wrote a detailed guide to 4e monster creation. Unfortunately my site is temporarily MIA, but I can email it.
Yeah, I noticed this when I started going through the MM comparing every monster to the design guidelines. Hit points, initiative, and (barring racial bonuses) skills are strictly according to formula, but defenses, attacks, and damage are all over the place.