well let me try and help you visualize why, Sorry if I get too carried away. But I know with me it helps to have a picture sometimes to understand things.
As you can see the track bar has two pivot points, one fixed to the frame (point B) and one mounted to the axle (point A). as the stock suspension sits, the track bar should be level to the ground and parallel with the axle. as the suspension moves through the range of travel point A moves upwards but also in an arc, using point B as the center of the arc. The length of the track bar is determined by the distance from point A and point B as the truck sits level and unloaded. So point A is at the longest point in the arc of travel even before there is any movement. as the suspension moves the axle will try and stay level regardless of the angle of the track bar. Thus as point A moves up and theoretically closer to point B the axle is forced to shift to the right. You have to remember that even though the true distance from point A to point B actually never changes (due to the fixed length of the track bar), the Vertical plain going through point A will 1) remain parallel to the axle plain and 2) shift closer to point B.
You mentioned that the stock suspension does not appear to shift when loaded so why does it do it when lowered. Well I bet it does, but just not enough that we would notice.
If you recall, we had to replace our bump stops with shorter ones with the drop kit. that's because the stock bump stops would not allow that much drop with out hitting the axle. So after the drop was done there is only about a 1/2" of shift to the right. I would bet that even with the stock suspension fully loaded the shift is there but, it is less than a 1/2 in. more like 1/4" in most cases. and I bet most people don't even think to look.
So really all we need to fix it is to have a track bar that is about 1/2" longer to push point A further away from point B, thus shifting the axle back to the right by 1/2". Or you could just drill a hole in the axle mount about 1/2" closer to point B, but i would be afraid that that would weaken the mounting point.
ANYWAY, sorry it got so winded. my head hurts now..lol