I believe with a bolt action you seat can the cartridge tighter—the tolerances are necessarily greater when you are mechanically trying to blow out the shell after each firing as with a semi-auto.
At least that’s what makes sense to me. A bolt is a much simpler and cleaner design and since speed of extraction is not an issue....you have the ability to have much tighter tolerances, which equals more accuracy.
Well, what you say does make sense...maybe that’s it after all.