The fact that you are not usually centered on the saddle could be caused by unconsciously compensating for (a) asymmetry in your legs such as leg length difference, or (b) asymmetry in your hips. I'd be willing to bet that both of the mentioned possible causes are fairly common in people.

I adjust for my asymmetry by slightly pointing the saddle to one side and it makes a huge difference in my comfort. Other people might adjust for their asymmetry by shifting their position on the saddle to one side or the other. Either way, your seated position should be one that is comfortable for you (as opposed to being text book perfect).