With standard stocks the main screw forward of the trigger tends to lock out through the trigger guard on the action lug with the stock inletted right through in the area of the lug. The stock is nipped to the action over the footprint of the trigger guard.
Custom stocks tend to be deeper in this area and one way or another require a longer action fixing screw and often the screw ends up pulling the entire underside of the action into the stock rather than just having a fixing relying on contact between the base of the trigger guard and the wood it rests against, controlled by the stock lug.
I'm fairly sure I had some big shift problems with my HW77/CS1000 caused by recoil induced flex of the long M7 main fixing screw that was used with no lug. It was unsupported over a fair amount of its length and the legacy of this flex could be seen in the backward smear of some mounting pads I experimented with between the stock and the action. Only on the shot movement of the action within the stock could cause this smear.
Strange thing is that I used the gun for years with little or no problem. After trying to 'improve' the inletting (smoothing it out) the problem emerged. I think the old imperfections may have previously prevented the action moving in the stock....