But they're also constructs...
And they're also living...
It's hard to really pinpoint what creature type the Modrons should be. Currently, with the Tertian modron, it's type is Construct (Extraplanar, Lawful).
This makes sense except for these problems:
1) Lack of weapon proficiencies
2) Medium base attack as opposed to High
3) Good Fortitude saves only instead of all good saves
4) 2 skill points/HD as opposed to 8
So I can see the dilemma. Problems 2 and 4 were offset thanks to a huge increase in HD with the Tertian and a huge increase in its Intelligence score. The increase in Intelligence actually suits them perfectly since they are a highly methodical and logical extraplanar race. It barely counters Problem 3, however, and doesn't solve Problem 1 but that's only a minor inconvenience.
The easiest route would've been Outsider type with construct traits. Dunno why it didn't go that route.