[rattlsnak]

Need to look at it from the other direction. It appears you're looking at from the suspension (alignment) outward but it doesnt work that way. You have to look at it from the tires inward. If a tire (any or all of them) is not sitting exactly square due to wear, (normal wear or uneven wear) then the suspension angles are altered. If there were no tires involved, then yes, once you set it, its a mechanical setting that doesnt change. But as a tire wears, the result of the angles changes.

In other words, lets say you measure a brand new car. Now drive it 50K miles and recheck the alignment. It will show off because of the tire wear. (even though no mechanical changes were made to the alignment settings. Now If you put new tires on it without making any mechanical changes to the settings, chances are you will be right back in spec. However if you make changes and THEN put new tires on, you will out of spec again as you have aligned the car the the worn tires.