This is the result of having excessive runout in the cue during assembly. Most likely, the faces of one or both sides of this joint were not faced properly. Causing the cue to have a fairly significant "wobble" or runout when it is screwed together.
Imagine the cue, when it was still much larger in diameter than finished size, spinning in the lathe and it has a big wobble in the middle. Possible as much as a quarter of an inch. Then with the cue spinning and over size, it is machined down to finish size. The high sides get cut down more so that they even out with the low sides and you have what appears to be a straight smooth well built cue. But, that screw that was thought to be spinning right in the center of the straight spinning cue is really swinging around in a circle.
Most likely, the cue will have a "heavy" spot that it will always want to roll to. If you put just the but on the table and slowly roll it and let it go, it may want to roll back and forth until it settles with that heavy spot of the pin closest to the bottom.
For this reason, many cue makers will machine all parts of the cue very close to finish dimension before assembling them together. They will also ensure that, when assembled, there is zero runout in the cue before making any final cuts. Other cue makers may still assemble their cue early, but will also ensure that there is zero runout before any cuts are made. Doing it this way does run the risk of some off center happening anyway because if the cue "moves" or does not stay dead straight during the turning process the same thing will happen.
Royce
