As often happens when I’m utterly certain about something, a particularly strong competing theory says I’m completely wrong.
Assuming that “level 6 skirmisher” gives the creature six actual “hit dice”, what we find is that subtracting three (half the creature’s levels) from the ability scores listed will return the standard 3e ability score modifier for the score given in brackets. In this case, the boosted modifiers explain that characters add half their level to ability score checks and untrained skill checks.
This explains the Init and Perception of +5, which means that Init is treated as a skill. Saves match a formula of 10 + full level + ability score modifier. Hit points match a much more traditional 5HP/level plus a con modifier of +2/level and a bonus +5 HP from either a Toughness feat or double HP at first level.
In other words, ignore everything my previous post said about base attack and changes to ability score modifiers and hit dice, or at least take it with a grain of salt.
What can we still deduce? In either case, advancing monsters is definitely going to be easier for the dungeon master. I’ve speculated that “Level 6 Skirmisher” isn’t necessarily just a descriptor – “skirmisher” could determine base attack, saves and hit points, just like a character class would. Will this play out consistently? Our spiky devil here appears to have save bonuses and base attack equal to full level along with 5HP/level, precisely in keeping with outsider hit dice (interestingly, HP are average-rounded-up just like my house rule).
On one hand, this seems to match the stats we already know, namely that outsider’s HP/saves are based on it being an outsider. On the other, monster roles being delineated to classes like “brute” and “skirmisher” make far more sense and allow for level to match hit dice without narrowing the scope of monster special abilities. It also allows for more straightforward and refined creature advancement, so in other words I imagine it to be the superior design decision. We’ll see how it turns out.