Would you please explain why a non-belted case would be more accurate than the same case without a belt?
There's two reasons.
With the larger diameter bolt face and case head belted cases have, that means any case with an out-of-square head can impact the bolt face further from center as the round fires. This means the same amount of force is now a greater distance from bolt face center than an equal capacity/length rimless case will produce. With the force further from center, receiver and barrel whip will be of a greater magnatude; the muzzle will be further away from its neutral position and the bullet will exit at a greater angle from the line of sight. That means it will strike further away from where it was aimed. This is the reason why `magnums' tend to throw a shot quite aways from group center every once in a while. Rimless cases don't do this nearly as often and when they do, the distance from group center that wide shot goes isn't nearly as far.
Belted cases are harder to make with very uniform body wall thicknesses. Rimless cases of the same length and capacity are typically more uniform in body wall thickness. When the body wall thickness is non-uniform around the case, the energy chamber is off-center from the chamber and the force is not directed backwards against the bolt face center as uniform as with rimless cases. This is the reason why PPC cases are favored for benchrest matches; their energy chamber is well centered on the chamber's axis due to their case wall thickness being very uniform all the way around the case.