First, he does not have a lowish average. It's a mere few points below Inzy.
And, it is harder to score centuries from six and below, because the final five wickets usually fall quicker, and for less runs, than the first five wickets. This is proven. Simply put, the players from eight and below are a lot worse than the others, ergo smaller quantity of reliable partner for number six batsman, ergo harder for number six batsman to score century. Easy maths.
This is why the likes of Hussey and Bell have usually been moved up from six, and why Laxman would have batted higher, had there not been many even bigger names stopping him from doing so. When good players are left at six they rarely have the time or means to impose themselves fully.
Remember many of Laxman's best innings came against a newish ball when the top four had failed anyway. You don't have his record at any position without being a very good player. In short, I don't think this part of your argument stands up very well.