Casualties, as opposed to KIAs, is misleading. A casualty can be just about anything - significant or insignificant. I know a lot of guys who have purple hearts for wounds that I am unaware of. And I know guys who have a purple heart for losing a couple of limbs. It's tough to made any judgments from figures on "casualties".
I think casualties have the closest statistical association with the level of enemy attacks because there are enough of them to smooth over random events. Our KIA, thank God, are too few to do so.
Unfortunately, the site I used does not say if they include wounds from non-combat accidents.