llSetLocalRot(), wandering object

Using llSetLocalRot() to rotate a child prim in a linked set appears to slightly alter the position of the entire linked set - sometimes. It seems like depending on where the object is, it gets better or worse. The change in position varies, its usually something like this: <0.00000, 0.00018, 0.00005>. Small amounts for sure, but they do accumulate and over time the object moves around substantially.

If I arbitrarily move the thing around, I can eventually find a place where it will just sit still when applying the rotation. I don't get it. Anyone help me board the clue train here?

Thanks, JL

just guessing but could it be that when you rotate that the difference in position changes the center of mass of the object and so it moves just a lil bit to center the object on the XYZ axis again?

I don't know why, but agree it does happen. One hack around it is to llGetPos on_rez or state_entry and then llSetPos() after each rotation. Less than satisfactory, but better than some of the alternatives!