If I've unlocked achievements "out of order" it's 'cause I've used a trainer or cheat engine or such. Which I do for every game I can.
I've completely given up doing all achievements honestly, if a game is bad or I just got bored of it, I'm quite happy using SAM to give myself 70%. The only games I won't do this for are games I've got a badge for; games I genuinely like or ones I've got 100%.
That damned "Avg Game Completion" stat has got me right in the OCD.