Had another idea whilst observing the spot... Noticed that when it expands, it gets a shadow along one side that looks like a neck shadow... which reminded me of how an ion trap works in a CRT.
Here's a picture of the yoke assembly from earlier:
There's no "traditional" ion trap, but there is a giant magnetic ring, with an inner sleeve which rotates by the grey knob behind the deflection coils... the focus assembly. Is it possible that on correct centering (the bright-dot is off center) the field is such that it aims the gun beam just off to the side when the deflection assembly isn't running?